Development of the Dermal Bones in the Skullof Lepidosteus osseus.

By

F. J. Aumonier, M.Sc,University College, London.

With Hates 1-8 and 2 Text-figures.

INTRODUCTION.

THE primitive features of the ' Ganoid' fishes have given thisgroup a position of considerable importance. Therefore itwas decided that the work described below would be amplyjustified for the sake of the observations alone, quite apartfrom the deductions which have been drawn from them.However, the main object of this work was an attempt tofind out whether the complex names of bones used by Sten-sio's school (post-rostro-nasal, inter-temporo-supra-temporal,&c.) were justified. That is to say whether some of thefusions which Stensio claims to have occurred in phylogenycan be found in ontogeny. Owing to the close relationshipexisting between certain of the dermal bones and the canalsof the latero-sensory system it seemed necessary to devoteparticular attention to the study of this relationship duringdevelopment. The nature of the association between boneand canal is unknown. It may well be that the presence of thesensory organs in the skin affects the underlying mesenchymedirectly, and induces it to lay down bone. Alternatively, thesame factor which determines the position of the sense organsmight simultaneously initiate a longer process which leads tothe formation of bone.

Pehrson (1921) published an account of the development ofthe skull of A mi a; this rendered it very desirable to discoverhow the development of L e p i d o s t e u s compared with itsless specialized relative. However, it is obvious that the greatdevelopment of the rostrum in L e p i d o s t e u s must bring

NO. 329 B

2 F. J. AUMONIEE

important functional demands upon the structure of the skull.Therefore allowance must be made for modifications both instructure and development, which have arisen chiefly to satisfymechanical necessities.

Agassiz (1833-43) gave the first detailed description of theadult skull of L e p i d o s t e u s . The accuracy of the figures andtext have not been equalled. Indeed it would seem as if laterworkers have concentrated on specialized aspects of morpho-logy, and thereby failed to appreciate the situation as a whole.

Parker (1882) described the development of the skull, butas at that time the importance of the lateral line canal was notunderstood, the value of his work is reduced. Moreover, thedermal bones are not so fully dealt with as are the cartilages andcartilage bones.

Allis (1905) in a general study of the latero-sensory canals offishes, dealt with L e p i d o s t e u s ; but, though he gave athorough account of the canals and pores, yet the dermal bonesreceived scant attention.

Veit (1907-11) described the chondrocranium in detail andextended his work with a thorough study of nerves and blood-vessels, but again there was no reference to dermal bones.

Eegan (1923) gave a short paper on the skull of Lepi-dos t eus . In this case the adult skull alone was described,though not in such intimate detail as had been given byAgassiz.

Mayhew (1924) published a somewhat longer paper on theadult skull of Lep idos t eus p l a t o s t o m u s . His work isof a more general nature than that of any preceding authorsince the time of Agassiz, but he does not deal with development.

Goodrich (1930) gives several figures of skulls of Lepi -dos t eus , but he was more concerned with special problemsof morphology than with the animal in particular.

Gregory (1933) deals very briefly with L e p i d o s t e u s andthe same remarks apply to his text and figures as to those ofGoodrich.

Hammarberg (1937) published a very extensive paper on theskull of L e p i d o s t e u s p l a t o s t o m u s . This is by far thebest work since that of Agassiz, and is well illustrated. There

SKULL OF LEPIDOSTEUS 3

are, however, certain differences between the observations ofHammarberg and those of the present author.

MATERIAL AND METHODS.

All except one of the specimens o fLep idos t eus osseusL.referred to in this paper were purchased from Turtox Ltd., ofAmerica, and had been fixed in formalin. The preservation wastolerable, but a certain amount of shrinkage had taken place.

Three specimens were examined and reconstructed: 19 mm.,25 mm., 38 mm.

The following stages were examined but not reconstructed:6 mm., 12 mm., 13 mm., 14 mm., 15 mm., 16 mm., 17 mm.,22 mm., 11 cm.

The specimens were decalcified in a solution of 4 per cent,nitric acid in 90 per cent, alcohol. After decalcification threephotographs were taken of each specimen (one dorsal, oneventral, and one lateral view). The overall length and the dis-tance from tip of snout to centre of the lens of the eye weremeasured and recorded.

The fish were then cut into serial transverse sections, 10/xthick for those of less than 25 mm. overall length and 20/i thickfor larger individuals. Eeconstructions were made at a magni-fication of 100 diameters for the smaller, or 50 diameters forthe larger, fish. Two views of each fish were reconstructed, onea plan, the other an elevation. In every case the plans representthe fish viewed from its dorsal surface, while the elevationsshow the left side of the animal. The reconstructions are all ofthe type of architectural drawings; no attempt is made toobtain perspective. This leads to a somewhat artificial appear-ance, but it enables accurate measurements to be taken withease.

The method employed was as follows: a microprojector wasset up so as to give the required magnification (either 50 or100 diameters). To obtain a 'plan' a sheet of 1 mm. squaredgraph paper was marked with a base line, every millimetredivision of which represented the thickness of one section. Theoutline of the animal and the position of the inner and outeredges of all bones are plotted with the aid of proportional

4 F. J. AUMONIBE

compasses set to divide by two. The distance between the twosides of the fish on a given section being set upon the outerends of the compasses, they are then transferred to the graphsheet and the distance between their inner ends (i :>. half thatof the outer) is then laid off on each side of the ' base line' alongthe line of the appropriate section.

This has the disadvantage of masking any slight asymmetryif it be present, but as such asymmetry would be devoid ofmorphological significance it was decided to accept this limita-tion for the sake of speed and ease of working. If the microtomecannot be relied upon to cut sections with reasonable accuracythere will be a longitudinal distortion; as, however, all struc-tures will be distorted equally, this also is of small account.It can most readily be detected by the shape of the orbit; ifthe latter appears on the graph as a circle, then for practicalpurposes there is no distortion.

The elevation is more difficult to obtain, but it is, if anything,more accurate than the plan. The photograph of the lateralview of the fish is projected on to a sheet of graph paper andthe size of the image is adjusted till the projected photographhas the same magnification as was used in plotting the sections.Taking a simple case, suppose the lens on the left side begins onsection 240 and ends on section 260, then its centre can beassumed to be on section 250; then, if the sections are 20^.thick, we use a magnification of 50 diameters for plotting, andso the photographic image is adjusted till the distance betweenthe tip of the snout and the centre of the lens is 250 mm. Inorder that the vertical lines of the graph shall correspond to theplane of section in a dorso-ventral direction a third point ofreference is required. Suppose the lower jaw first appears onsection 83. Then our graph sheet is moved about till the tip ofthe nose lies on division 1, the centre of the lens on division 250,and the tip of the lower jaw on division 83. The orientation isnow as exact as may be, and the outline of the fish is then tracedin pencil on the graph paper. Other easily identifiable structuressuch as the lens, operculum, nostrils, gape, &c, are also traced.Owing to the flexibility of the floor of the mouth there is boundto be a considerable discrepancy between the depth of the fish

SKULL OF LBPIDOSTBUS 5

before embedding and after cutting. For this reason the outlineof the dorsal surface is used as a datum line and is at onceinked in. The image of the sections is projected in the samemanner as in making the plan; but, instead of using propor-tional compasses, the distance between the dorsal surface andthe upper and lower limits of all bones is transferred from theimage of the section to the graph by means of dividers. Ifthe ventral surface be plotted thus, it will be noticed that theplotted outline is always more dorsal than the outline obtainedfrom the photograph; and this difference increases towards theposterior end.

The important stage of 25 mm. was the only exception to theabove procedure. This was a series of 10/z. sections very kindlylent by Dr. G. E. de Beer. As there was no photograph of thisspecimen, the elevation was obtained by assuming that theparasphenoid was a straight line. After a preliminary outlinehad been obtained, the outline of the dorsal surface was fairedby eye till it assumed a shape roughly similar to that of one ofthe adult skulls which was available. The parasphenoid wasthen replotted, using the revised dorsal surface as a datum line.The outline of the head anterior to the parasphenoid was recon-structed by Pusey's method.

I shall take this opportunity for thanking Professor D. M. S.Watson and Dr. E. A. Fraser, under whose supervision the workwas done (at University College, London), for much helpfulcriticism, and Dr. G. R. de Beer, for the loan of the 25-min. stage.Dr. T. S. Westoll has also given me good advice on many points.Mr. H. K. Pusey very kindly showed me how to use his systemof reconstruction, which was essential for the elevation of theanterior end of the specimen lent by Dr. de Beer. I am indebtedto Mr. J. R. Thomas for the photomicrographs and for thephotographs of the specimens; these photographs were anessential part of the method of reconstruction. Mr. S. R. Scarfehas advised me in the preparation of sections, and Miss JoyceTownend in the preparation of the figures.

The question of bone nomenclature must next be considered.Owing to the rapid advances now being made in the study ofmorphology, it is desirable to present observations in such a

6 P. J. AUMONIBR

manner that the reader can readily separate the facts from thetheories. Therefore the following method will be adopted. Thedevelopment of the skull will be described bone by bone, eachelement having a separate paragraph. The paragrapl will beheaded with the original French terms used by Agassiz, and thename which the present author believes to be correct will followin brackets. These latter names will be used in the text and onthe figures. In the discussion the bones will be dealt with ingroups and the author will attempt to justify his choice ofterms.

In order to assist any one who may wish to follow moremodern authors, the following table gives the French terms ofAgassiz and their modern equivalents. In most cases the namesof some of the modern authors are given in brackets. This listdoes not pretend to be complete, but it may be a useful guide.

Terms used by Agassiz.

Frontaux principaux.Frontaux anterieurs.

Nasaux.

Frontaux posterieurs.Basilaire.Parietaux.Sphenoide principal.

Oooipitaux superieurs.

Oocipitaux externes.Oocipitaux lateraux.Grandes ailes du Sphenoide.

Mastoidiens.

Rochers.Ailes orbitaires.Ethmoiide cranien.Vorncr.

Intermaxillaires.

Maxillaires superieurs.Jiigaux.

Modern Terms.

Frontal.Prefrontal.

(Premaxilla (Gregory).( Ethmonasal (Parker).VNaso-premaxillary (Mayhew).Sphenotic (Regan, Mayhew).Basioocipital.Parietal.Basisphenoid.

I Dermo-supra-occipital (Gregory).( Dermo-ocoipital (Mayhew, Regan).(.Post-parietal (Goodrich).Tabular (Mayhew, Gregory).Exoooipital (Mayhew, Regan).Pro-otic (Regan, Mayhew).

/ Supra-temporal (Mayhew).\ Pterotic (Goodrich, Regan, Gregory).Epiotic (Mayhew, Parker, Veit).Alisphenoid (Mayhew).Orbitosphenoid (Mayhew).Vomer (Regan, Mayhew).

I Premaxillary (Gregory, Regan, Goodrich).1 Nasopremaxillary (Mayhew).

Maxillary chain.Preorbitals (Mayhew).

SKULL OF LEPIDOSTEUS 7

/These include Mayhew's dermethmoid,I median prenasal, and lateral prenasal.

Os mobiles du nez. { Goodrich's ethmoid, nasal, and adnasal.I Gregory's nasal, adnasal (or antorbital) andV dermethmoid (or rostral).

Palatins f Ectopterygoid (Regan).\ Palatopterygoid (Mayhew).

Transverses. Pterygoid (Ectopterygoid) (Gregory).Pterygoides. Metapterygoid.Os cane. Quadrate.Caisse. Sympleetic (Mayhew, Regan).Operculaires. Opereular (Mayhew, Regan).„ , , (Preoperoular (Mavhew, Goodrich).Preopercules. \ T , , .-1 .

r I Interoperculum (Regan).(Interoperculum (Gregory).

Tympano-malleaux. | Interoperculum (Goodrich).{ Praeoperculum (Regan).

' a, b, c ' Mentioned by Mayhew.'p, p.' Not mentioned by any other author.Sous-orbitaires ( f a n ) Circumorbitals.

Before dealing with the dermal bones of the skull, it will benecessary briefly to describe the lateral-line sense organs, andalso to give some account of the chondrocranium. The individualsense organs of the latero-sensory system of L e p i d o s t e u sare developed early. They were found in the 12-mm. stage, andthe only difference between the organs found in this fish andthose of the 19-mm. stage was that of size. The appearance ofthe organs in a transverse section of the fish's head is verysimilar to that of a sagittal section of an onion. Each organconsists of a bundle of thin columnar cells, which extend fromthe body surface to the basement membrane. The bundles arealmost spherical, but in the centre of each a narrow round-bottomed depression of the body surface extends downwardstowards the centre of the organ. This depression varies indepth from one organ to another, but is generally from one-halfto one-third of the total depth of the organ. When the canalsare formed the organs become much larger and flatter, whilethe central depression vanishes.

The lateral-line organs can be distinguished from the othercutaneous sense organs by the central depression and by theirlarger size; also by the fact that when the reconstruction is

8 F. 3. AUMONIER

made they form smooth continuous lines. Their number issubject to some individual variation, as the following table willshow:

19 mm. 25 mm. 38 mm.Number of organs in supra-orbital line . 28 21 25Number of organs in infra-orbital line 32 32 25Number of organs in operculo-mandibular line 25 24 21

There is no indication of any increase in number after the19-mm. stage.

Text-fig. 1 is a photomicrograph of an organ from the supra-orbital line of the 19-mm. fish; Text-fig. 2 is of an organ in thecanalized portion of the infra-orbital line of the 38-mm. fish;both taken with Zeiss 3-mm. apo. objective N.A. 1-4 and drycondenser (below).

TEXT-FIGS. 1-2.

Photomicrographs. Both are taken with Zeiss apochromatic 3 mm.N.A. 1-4 and dry condenser.

Fig. 1.—Organ from supra-orbital line of 19-mm. stage.Fig. 2.—Organ from canalized region of infra-orbital line of

38-mm. stage.

The chondrocranium has been so well described by Veit(1907 and 1911, and Hammarberg, 1937) that only the briefestaccount of one stage (38 mm.) will be given here. Fig. 22,PI. 7 is a dorsal, fig. 23, PL 7 a lateral, and fig. 24, PI. 8 a

SKULL OFLEPIBOSTBUS 9

ventral view of the chondrocranium of the 38-mm. fish. Fig.24, PL 8 is a true ventral view and not a dorsal view ofelements on the ventral surface as is the case in the palatalreconstructions of the dermal bones. In fig. 22, PL 7 a smallsection of the supra-orbital bar on the right hand side has beenremoved to show the articulation of the palatopterygoquadratecartilage with the basitrabecular process. The trabecular hornon the same side has also been removed to show the palatalprocess of the palatopterygoquadrate cartilage.

In fig. 23, PL 7 the trabecular horn is only partly visible intwo places owing to the presence of the palatal process of thepalatopterygoquadrate. On the dorsal surface the left-handside of the posterior fontanelle and of the taenia tecti mediaposterior are visible. The anterior fontanelle is hidden. Infig. 24, PL 8 the right-hand side has had the palatopterygo-quadrate and hyomandibula removed, in order to show thecornu trabeculae and the basitrabecular process. The furtherdetails of the arrangements of the cartilages can be seen froma study of the three figures showing reconstruction of cartilageand bone (figs. 25, 26, and 27, PL 8), and from the numeroustransverse sections of the different stages.

The following is a description of the bones.

Os Mobiles du Nez. (Eostrals, nasals, and antorbitals.)There are three pairs of these, all of which are superficial.

The most anterior pair lies on the ventral surface anterior tothe olfactory organ. It was only seen in the 38-mm. fish andwas very small; owing to the oblique angle at which the skin ofthe snout was cut, it was impossible to identify the ethmoidcommissure, but a few lateral-line organs (not shown in thefigures) seem to be associated with this anterior pair of bones.The bones are shown in plan in fig. 14, PL 6, in elevation infig. 13, PL 6, and in transverse section in fig. 12 a, PL 5. Theirposition relative to the rostral cartilage is shown in figs. 26and 27, PL 8.

The next pair lies on the dorsal surface. In the 38-mm. stagethe bones cover the top of the olfactory organs, extending froma point vertically above the anterior end of the rostral cartilage

10 F. J. AUMONIBE

(septum rostri of Veit) to a position abreast of but mesial to theposterior nostrils. The right-hand bone has three, the left five,organs of the supra-orbital line associated with it. In the 38-mm.stage they are illustrated in fig. 13 (elevation), fig, 14, PI. 6(plan), in transverse section in fig. 126, PL 5, and in relation tocartilage in fig. 25, PI. 8 (plan) and fig. 26, PI. 8 (elevation). Thesebones were first seen in the 25-mm. specimen. They were verysmall and lay near the middle of the inner edge of the olfactoryorgans, in intimate association with the most mesial pair of organsof the supra-orbital lines, fig. 6, PL 2 and fig. 7, PL 3 (elevation),fig. 8, PL 3 (plan), and fig. 6a, PL 2 (transverse section).

The third pair has only been seen in the 38-mm. stage. Thebones lie laterally to the posterior end of the olfactory organ ;each bone carries two organs of the infra-orbital line. Fig. 13,PL 6 shows the bones in elevation, fig. 14, PL 6 is the plan;the transverse section is seen in fig. 12 c, PL 5, and the relationsto the septum rostri in figs. 26 and 27, PL 8.

It should be noted that no signs of cartilaginous olfactorycapsules could be found at any stage.

I n t e r m a x i l l a i r e s .It was found that these were really the anterior ends of the

bones called 'nasaux', and even in the 17-mm. stage there wasno sign of a separate origin.

N a s a u x . (Premaxillaries.)These bones were first seen at the 14-mm. stage. In the

19-mm. fish they are of large size. They lie one on each side ofthe septum rostri, and are thin sheets of bone which are flatand lie vertically at their anterior ends; but, immediatelyposterior to the anterior nostrils, they become strongly curvedin transverse section (with the concavity facing laterally). Atabout the middle of their length a lateral shelf is formed andthe olfactory nerve runs ventrally to it. In later stages the outeredge of the shelf extends ventrally and fuses with the mainbone ventral to the olfactory nerve, which is then enclosed ina bony tunnel. The posterior extent of the bones varies with thesize of the fish, but though in the 19-mm. stage there is a gap

SKULL OF LEPIDOSTEUS 11

between the posterior ends of the premaxillaries and the anteriorends of the frontals, yet these two pairs of bones overlap in the25 mm. and all subsequent stages. The premaxillaries alwayslie very close to the septum rostri, but they do not come intoany kind of association with the organs of the supra-orbital lineuntil the horizontal shelf mentioned above has been establishedfor a long time. In the 19-mm. stage they can be seen infigs. 1 and 2, PI. 1 (elevation and plan), figs. 1 a and b, PL 1(transverse section). In the 25-mm. fish, fig. 6, PL 2 and fig. 7,PL 3 are elevations, fig. 8, PL 3 is a plan, and figs. 6 a, b, c, andd, PL 2, are transverse sections at different stations. Fig. 6 d,PL 2 shows the overlap of the premaxillaries and frontals. Theillustrations of the 38-mm. specimen show the premaxillariesin fig. 12, PL 4 (elevation), fig. 14, PI. 6 (plan), figs. 12 c, d,e, f, and g, PL 5 (transverse sections). Their relations to thecartilages are shown in the sections and in figs. 25 (plan) and26, PL 8 (elevation).

F r o n t a u x P r i n c i p a u x . (Frontals.)These were first observed in the 16-nnn. stage. They were

small, slightly concave plates lying below the organs of thesupra-orbital line for a distance of three consecutive organs justabove the orbit. The bone was slightly thicker in the region ofthe organs than in between them. It was, however, continuousthroughout, and was thickest under the middle organ, thinneranteriorly, much thinner posteriorly. In the 19-mm. fish thefrontals were quite large bones. Their relationships with thepremaxillaries have been described above. Posteriorly theyextend slowly and do not reach far beyond the orbit till the38-mm. stage when they carry 12 organs. They lie very closeto the supra-orbital cartilage, slightly lateral to it. In the 38-mm. stage their posterior ends are flared outwards so as to passoutside the antero-lateral edges of the posterior fontanelle.They lie very near to the cartilage at all stages. Fig. 1, PL 1is a lateral and fig. 2, PL 1 a dorsal view of these bones in the19-mm. specimen, the transverse section is seen in fig. 1 c, PL 1.In the 25-mm. stage they are shown in elevation in fig. 6, PL 2and fig. 7, PL 3, in plan in fig. 8, PL 3, and in transverse

12 F. J. AUMONIBR

section in figs. 6 d, e,f, and g, PI. 2. The figures for the 38-mm.fish are 12, PI. 4; 14, PL 6; 12 g, h, i,j, k, PI. 5.

P a r i e t a u x . (Parietals.)The posterior end of the skull develops somewhat later than

the anterior, and these bones were first seen in the 38-mm. stage.They are simple sheets of bone lying near the mid-line on thedorsal surface and extending from the posterior end of the orbitto the middle of the auditory capsule. Their antero-lateraledges just overlap the postero-mesial borders of the frontals.Their mesial borders do not quite reach the lateral margins ofthe taenia tecti media posterior, thus the posterior fontanelleis still incompletely roofed at this stage. The posterior ends ofthe parietals do not reach the posterior end of the auditorycapsule; the lateral edges do not descend very far down thesides of the cranium. Fig. 12, PI. 4 gives the elevation and fig.14, PI. 6 the plan of the parietals; the sectional view is fig.12fc, PL 5, and the relations to the cartilage can be seen infigs. 25 (plan) and 26, PL 8 (elevation).

The posterior row of roofing bones and the shoulder girdlewill be dealt with later.

Maxi l l a i res S u p e r i e u r s . (Infra-orbitals.)The first indication of these was in the 25-mm. stage. In this

animal they were small gutters immediately underlying eachorgan of the infra-orbital line. The series began with the firstorgan posterior to the posterior nostril and extended for fiveorgans in succession. The mesenchyme condensations in whichthey were found were in close contact with the epidermis in theregion of each organ. The bones appear to develop from beforebackwards, the posterior elements being very small. A fewteeth had become attached to the larger bones. A great numberof teeth have no attachments of any sort till much later.

In the 38-mm. specimen, the bones had become more elabo-rate and had increased in size. The most anterior element ofeach row had fused with the one behind it, while the gapsbetween the succeeding bones were reduced to narrow slits.The infra-orbital line ran through the first five elements as a

SKULL OF LEPIDQSTEUS 18

closed canal with pores at each suture. Many teeth wereattached to the bones (fig. 6, PL 2; figs. 7, 8, PL 3; fig. 6 b,PI. 2; figs. 13, 14, PL 6; figs. 12d, e, and/, PI. 5).

'a, b, c ' (Maxillaries.)

It has not been possible to establish the existence of threebones in this region. Only one could be found, and this wasalso the case in all four of the adult skulls studied. The singlebone is present in the 17-mm. and all subsequent stages: it liesin a fold of skin between the upper margin of the gape and thelower margin of the labial groove. Its anterior end is betweenthe fifth and sixth organs of the infra-orbital line (but farventral to them). It reaches its maximum relative size in the25-mm. stage and is relatively smaller at 38 mm. In every caseit bears a few teeth; three or four small specks of enamel werefound on this bone in a large adult skull. These might be theremains of teeth formed in early development which hadremained vestigial (figs. 1, 2, 2b, PL 1; fig. 6, PL 2; figs. 7, 8,9, PL 3; figs. 6b and c, PL 2; figs. 13, 14, PL 6; fig. 12g, PL 5).

S o u s - o r b i t a i r e s . (Infra-orbitals.)There is no sharp division between the ' sous-orbitaires' and

the 'maxillaires superieurs'. Indeed there may not be any ofthem present in the 38-mm. fish. If they do exist, they are thetwo most posterior elements of the chain labelled infra-orbitalsin fig. 13, PL 6 (those in which the infra-orbital line is a deepgroove but no longer a closed canal). They are superficial andthe infra-orbital hue passes over them. Lamellae of bone riseup beside the groove.

It may be mentioned here that the cheek plates are absentin the 38-mm. stage.

P r e o p e r c u l e . (Preopercular.)This lies in the opercular flap and is quite superficial. It was

first seen in the 25-mm. fish. The operculo-mandibular lineruns along it, but only two organs come into intimate relationwith the bone. In the 38-mm. specimen the bones are much

14 F. J. AUMONIBR

larger, and each one carries six organs (fig. 6, PI. 2; figs. 7,8, (> h and i, PI. 3; fig. 12, PI. 4; fig. 17, PI. 7; figs. 12 j and k,PL 5).

It will be seen that the preopercular covers the ventra1 andposterior regions of the hyomandibula (fig. 26, PI. 3).

Opercu les . (Opercular.)These bones were first seen as small thin sheets in the 38-mni.

stage. They lay in the opercular flaps, slightly above and behindthe postero-dorsal corner of the 'preopercules', between thelatter bones and the supratemporals. They have no connexionwith any part of the latero-sensory system (fig. 12, PI. 4; fig.17, PI. 7).

S o u s - o p e r c u 1 e s. (Subopercular.)This again was first seen in the 38 mm. It lies ventrally to

the opercular, and the same remarks apply to it as to theopercular (fig. 12, PI. 4; fig. 17, PL 7).

M a s t o I d i e n s. (Supra-temporals.)In the 25-mm. stage these bones were quite small but they

had a recognizable association with the infra-orbital line (eachbone carried one organ). They are superficial, and are situateddorso-laterally, directly above the preopercular. In the 38-mm.fish they are larger and carry four organs of the infra-orbitalline. They are lateral to and coextensive with the parietals:their antero-dorsal edges overlap the postero-ventral marginsof the frontals. The antero-ventral borders of the supra-temporals do not quite reach the ventral margins of the supra-orbital cartilages. At about the middle of their length theventral edges of the supra-temporals overlap the articulationof the hyomandibula with the side of the auditory capsule.

Their elevation is shown in fig. 6, PI. 2, and fig. 7, PI. 3 forthe 25-min. and fig. 12, PI. 5 for the 38-mm. stage. In plan,fig. H, PI. 8 gives the situation at 25 mm. and fig. 14, PI. (! at38 mm. The transverse sections are shown in fig. (ii, PI. 3.

The relations to cartilage appear in ligs. 25 (plan) and 26,PI. 8 (elevation).

SKULL OFLEPIDOSTBUS 15

P a l a t a l Bones .

Vomers . (Prevomers.)The prevomers appear at the 14-mm. stage, and in the

19-mm. specimen have attained a reasonable size. They areimmediately ventral to the septum rostri. Their anterior endsare just posterior to the anterior nostrils, while posteriorly theypass one on each side of the parasphenoid. In the later stagesthey are much bigger: they grow posteriorly and laterally.Their inner edges almost meet in the mid-line just ventral tothe parasphenoid. They bear many teeth. They are alwaystightly applied to the ventral surface of the septum rostri.In the 19-mm. specimen they are shown in figs. 2 (plan), 5(elevation), and 1 a (transverse section), PL 1. For the 25-mm.stage the figs, are 8 and 9 (plans), 10 (elevation), PI. 3, figs.6 b and c, PI. 2 (transverse sections). The prevomers of the38-mm. fish are shown in fig. 15, PI. 6 (elevation), fig. 17, PI. 7(plan), figs. 12 d, e, f, PI. 5 (transverse sections). The neigh-bouring cartilages are seen in figs. 26 (elevation) and 27,PL 8 (ventral view). It must be mentioned that fig. 27, PL 8is the only exception to the practice of showing the palatalbones as viewed from the dorsal surface.

P a 1 a t i n s. (Palatines and ectopterygoids.)Agassiz regarded the ectopterygoid as the palatine and

thought that the thin tooth-bearing bone which lies ventrallyto its anterior end ('superficial palatine' of Parker) was anaccessory tooth-plate, comparable to those in the oesophagus.These superficial palatines were found in all stages from 14 mm.onwards. In the 19-mm. specimen the posterior half of thepalatines lies just ventrally to the anterior ends of the ecto-pterygoids, but in later stages the ectopterygoids extendanteriorly till only the extreme tips of the palatines extendbeyond them. The palatines are illustrated in figs. 2 (plan),5 (elevation), 1 a and b, PL 1 (transverse sections), for the19-mm. fish. Figs. 8 (plan), 10, PL 3 (elevation), fig. 6 c, PL 2(transverse section) show the 25-mm. specimen, while fig. 16, PL

16 F. J. AUMONIBR

6 (elevation), fig. 17, PL 7 (plan), and fig. 12/, PI. 5 (transversesection) are of the 38-mm. stage. The cartilages and theirrelations to the palatine are shown in figs. 26 (elevation) and27, PL 8 (ventral view).

The ectopterygoids are also developed early, making theirappearance in the 14-mm. fish. Their relationship to thecartilaginous skeleton is interesting. In the centre of the snoutlies the long slender cartilaginous septum rostri. In the 38-mm.stage the cornu trabeculae have fused with the septum rostrifor the posterior third of its length, for the middle third of thecourse of the septum rostri the cornua are free forwardly pro-jecting rods. Immediately ventro-laterally to the cornua liethe slender palatal processes of the palatopterygoquadratecartilages. Tightly pressed to the ventral surfaces of the palatalprocesses are the ectopterygoids. In this stage they are thickcancellated masses which must have an important role instiffening the snout. At the level of the anterior end of thebrain case the palatopterygoquadrates take on a differentform. They are here deep girders, the dorsal edges of which arenearer the mid-line than the ventral. The antero-ventral cornuaof the palatopterygoquadrates project slightly forward andform the articulation for the lower jaw (hence the lengthy nameof palatopterygoquadrate is justified). At the point where thepalatal process joins the main girder, the ectopterygoid becomesa broad thin sheet and is tightly pressed to the median face ofthe palatopterygoquadrate. The bones are shown in the 19-mm.stage in figs. 2 (plan), 5 (elevation), and 1 b and c (transversesections), PL 1. The illustrations for the 25-mm. fish are fig.6, PL 2 (elevation), fig. 8, PL 3 (plan), and figs. 6 c, d, e, / ,and g, PL 2 (transverse sections). The 38-mm. specimen isshown in fig. 16, PL 6 (elevation), fig. 17, PL 7 (plan), andfigs. 12/, g, li, i, j , and k, PL 5 (transverse sections). Therelations to the cartilages are best seen in the transversesections, but figs. 26 (elevation) and 27, PL 8 (ventral view)may be useful.

P t e r y g o i d e s. (Endopterygoid.)According to Agassiz the bone since called by Mayhew the

SKULL OF LBPIBOSTBUS 17

metapterygoid was the ' pterygoiide', while the mesopterygoldof Mayhew was a mere accessory plate. The cartilage bone doesnot concern us, but the accessory plate exists as a thin splintrunning along the dorso median edge of the pterygoid portionof the palatopterygoquadrate for a short distance below theposterior half of the orbit in the 38-mm. stage. Fig. 12, PI. 5(elevation), fig. 17, PI. 7 (plan), fig. 12 j , PL 5 (transverse sec-tion), figs. 26 (elevation), and 27, PI. 8 (ventral view).

Sphenoi 'de P r i n c i p a l . (Parasphenoid.)This bone is first seen at the 14-mm. stage. In the three stages

which have been reconstructed, it is of great length. In the19-mm. fish it already extends from the posterior third of theprevomer to the extreme posterior end of the skull. In the laterstages it extends steadily forwards. Pigs. 2 (plan), 5 (eleva-tion), and 1 a, b, and c, PI. 1 (transverse sections) show theparasphenoid in the 19-mm. stage. It has not been shown infull in the 25-mm. stage but figs. 8 and 9, PL 3 show the pos-terior and anterior ends respectively in plan, while fig. 6, PL 2and fig. 7, PL 3 show the posterior end in elevation. In the38-mm. specimen the parasphenoid is seen in elevation in fig.15, PL 6, in plan in fig. 17, PL 7, and in transverse section infigs. 12/ to k inclusive, PL 5. Its relations to the chondro-cranium are seen in figs. 26 (elevation) and 27, PL 8 (ventralview). The parasphenoid is always pressed closely to theventral surface of the chondrocranium, from the septum rostrito the occipital region except in the region of the basicranialfenestra, which it occludes in the 38-mm. stage.

T y m p a n o - m a l l e a u x . (Suborbitals.)These are present in the 25- and 38-mm. stages. They lie

along the ventro-lateral edges of the quadrate portion of thepalatopterygoquadrate cartilages, extending from just posteriorto the articulation of the lower jaw to the level of the basi-pterygoid process. In the 38-mm. fish the anterior part of thepreopercular overlaps the posterior half of the suborbital. Inthe 25-mm. stage the suborbitals are shown in fig. 7, PL 3(elevation), fig. 11, PL 4 (plan), and fig. 6i, PL 3 (transverse

NO. 329 C

18 F. J. AUMONIER

section). In the 38-mm. fish they are visible in fig. 12, PI. 4(elevation), fig. 17, PI. 7 (plan), figs. 12 i and j , PI. 5 (transversesections). Their relations to the cartilages are seen in figs. 26(elevation), and 27, PL 8 (ventral view).

Lower J a w .D e n t a i r e s . (Dentaries.)

This term includes both dentary and coronoids, for Agassizdid not realize that the complementaries were present. Thedentary was well developed in all stages from 14 mm. onwards.It is a stout sheet of bone which lies vertically just laterally toMeckel's cartilage. Its lower edge is bent inward to form ahorizontal shelf which lies ventrally to Meckel's cartilage. Thedorsal edge of the dentary bears many large teeth.

Hammarberg describes the dentary as a compound boneresulting from the fusion of a true dentary with three lateral-line bones. My observations are in agreement with this, exceptthat only one lateral line component could be found. In twoother places mesenchyme condensations similar to thosedescribed by Hammarberg were found, but in them bone-formation took the form of an outgrowth from the dentary andnot an independent spicule.

In the 19-mm. stage the dentary is seen in figs. 1 (elevation),3 (plan), and 1 a and b, PL 1 (transverse sections). For the25-mm. stage the figs, are 7 and 10, PL 3 (elevation), fig. 11,PL 4 (plan), figs. 6 a-e inclusive, PL 2 (transverse sections).

The coronoids develop later: the posterior coronoid alonewas present in the 25-mm. stage, while a small anterior coronoidwas present in the 38-mm. fish. The coronoids are immediatelydorsal to Meckel's cartilage and carry numerous teeth. Theyare shown in the same figures as the dentary.

Angu la i r e . (Angular and articular.)This again includes two bones, Mayhew's angular and derm-

articular. The latter is just recognizable in the 17-mm. specimenand in the 19-mm. stage has reached a considerable size(figs. 1 (elevation), 3 (plan), 1 b and c, PL 1 (transverse sections)).

SKULL OF LEPIDOSTBUS 19

In the 25-mm. fish it projects posteriorly to the dentary,figs. 7, 10, PI. 3 (elevation), fig. 11, PL 4 (plan), figs. &d ande, PI. 2 (transverse sections). The dermartioular lies justlaterally to the posterior end of Meckel's cartilage, and its longanterior end lies Between Meckel's cartilage and the ventralhorizontal shelf of the dentary. These features are morepronounced at 38 mm. For the latter stage see fig. 12, PL 4(elevation), fig. 19, PL 7 (plan), figs. 12 e and / , PL a (trans-verse sections).

Mayhew's angular first appears in the 25-mm. fish, and isslightly larger at 38 mm. (figs, as above). It is closely appliedto the extreme postero-ventral cornu of Meckel's cartilage.It has no relation to the lateral-line system and in the 11-cm.stage fuses with a cartilage bone.

O p e r c u l a i r e . (Prearticular.)This bone was only seen in the 38-mm. fish; it is a large thin

flake on the lingual side of the posterior end of Meckel's cartilage(figs. 18, 19, PL 7; fig. 12 g, PL 5).

DISCUSSION.

When an attempt is made to arrive at a reasonable nomen-clature for these bones, many difficulties arise. In giving thepresent writer's views as to the correct equivalents for theFrench terms used in the foregoing descriptions, it will probablybe best to deal with the bones in groups, as suggested above.

BONES RELATED TO THE SUPB A-ORBITAL LlNE.

The most anterior of the bones of the supra-orbital line, arethe median dorsal pair of the ' Os mobiles du nez'. These arethe nasals. They are much reduced and first appear immediatelyunderlying the most mesial pair of organs of the supra-orbitalline. They are best shown in fig. 8, PL 3, which is a dorsal viewof the 25-mm. fish: the roofing bones are seen on the fish'sright and the palatal bones on the left. (It should be stated thatthe bone and its associated organ are more anterior on theanimal's left side than on the right, therefore in fig. 6, PL 2and fig. 7, PL 3 section ' a' passes through the organ, while in

20 F. J. AUMONIEB

fig. 8, PL 3 section ' a ' passes anterior to the bone.) Unlikethe nasal of A mi a (Pehrson, 1922), this bone has a singleorigin; a compound name is not therefore justified for the nasalof L e p i d o s t e u s .

Hammarberg states that the nasal of L e p i d o s t e u s isdeveloped from three distinct rudiments, one of which is paired.The whole development of this bone is stated to be the same asthat of the nasal of A mi a (described by Pehrson). In Lep i -dos t eus osseus , however, it is most obvious that no suchprocess occurs. The nasal is a single bone, from the earliest tothe latest stage examined. It is probable, therefore, that thedifferent species of L e p i d o s t e u s have different modes ofdevelopment. The pairing of the bone rudiments is also absentin Lep idos t eus osseus . It is true that individual sectionsof a series can be found which show such a pairing, but in allcases the division is so short as to be insignificant and may notbe similar on the opposite side of the fish. This pairing is almostcertainly nothing more than the provision of nerve foramina.Hammarberg thinks that the pairing of the rudiments of thenasals is due (in L e p i d o s t e u s p l a t y s t o m u s ) to the factthat the bone ridges which later form the tube of the lateral-line canal, are laid down in the position where the two membersof the pair first appear: ' On that account it seems to me quitenatural that during development those parts of it first appearwhich are thickest in the finished bone.' In L e p i d o s t e u sosseus , however, the ridges appear much later and hencethis argument is not valid.

In the 38-mm. stage the bone has increased in size so as tocover the greater part of the olfactory organ, and now carriesfour organs of the supra-orbital line.

The next bones in the supra-orbital series are those labelled'nasaux'. They are large bones lying one on each side of therostral cartilage. These bones in the adult carry a considerableportion of the supra-orbital canals; but in the young stages itis obvious that this association is a secondary condition. Theantero-ventral extremities of the 'nasaux' carry teeth, and therecan be little doubt that they are the premaxillaries. They showno trace of a compound origin at any stage.

SKULL OF LEPIDOSTEUS 21

Hammarberg states that the premaxillary is a compoundbone which consists of a premaxillary and two nasals. In myspecimens no such arrangement can be justified. Hammarbergfound a mesenchyme rudiment extending from the ventro-lateral edge of the premaxillary to the lateral-line canal justposterior to the olfactory organ. This condensation was alsofound in my stages, but in the 15-mm. fish, although it waspresent on the edge of the premaxillary, it failed to reach thelateral-line organs. Furthermore, in most stages it was denserat the ventral end than at the dorsal end. For these reasonsI maintain firstly, that this condensation begins from the pre-maxillary and is secondarily associated with the lateral line,and secondly, that mesenchyme condensations are not a verysafe guide unless supported by other evidence. In no case werethe independent centres mentioned by Hammarberg found tobe separated when actual bone was present. It is probable thatthe explanation of this extraordinary development of the pre-maxillaries is mechanical necessity. The snout needs supportagainst the stresses set up by biting, which are in a dorso-ventral plane. A thin sheet of bone placed on edge would meetthe requirements of these stresses. As the premaxillary hasa posterior process in E u g n a t h u s , Amia , and Lepi -d o t u s, it is not surprising that this process has been extendedin L e p i d o s t e u s in such a fashion as to stiffen the anteriorpart of the snout. The peculiar transverse curve of the posteriorprocess enables it to withstand considerable lateral stress, andalso to lie closely against the rostral cartilage. Since this boneextends to the dorsal surface at an early stage, there is nonecessity for the nasal to extend posteriorly, and the premaxil-lary finally comes to enclose the lateral-line canal, but only ata later stage than any of the specimens examined. Thisexample of a secondary association between a bone and alateral-line canal is interesting, but it is probable that such acondition is rare and only takes place under conditions ofmechanical necessity.

Posteriorly to the premaxillaries come the frontals. Pehrsondescribes the frontals of Amia, and says that they develop inthree parts each of which is in turn of a double origin. In

22 F. J. AUMONIEE

Lep idos t eus there is no evidence of the three main com-ponents; and clear evidence against the longitudinal divisionof the bones. Pehrson maintains that osteoblast rudiments arelaid down on either side of the organs of the lateral line canal,and that the bones are thus laid down in two lateral parts. InL e p i d o s t e u s , on the other hand, the osteoblast rudimentsare always directly below the sense organs, and the bone thushas a single origin. In the 19-mm. stage the anterior ends ofthe frontals are forked as Pehrson described them in A m i a,and this lends a clue to the explanation of the differences indevelopment. In A mi a the supra-orbital line is canalized atan early stage, while the organs lie at the bottom of a fairlydeep groove before bone-formation starts. This leaves little spacebetween the bottom of the groove and the chondrocranium,hence the mesenchyme condensation is cut in two and forcedlaterally. In L e p i d o s t e u s , on the other hand, the senseorgans remain at the general surface level until bone develop-ment has begun, and the grooves are only just becoming deepat 19 mm. Therefore there is no mechanical interference withthe normal course of events, and the bones are laid down insingle shallow gutters; when, however, the groove deepens andsets up pressure, the developing anterior ends become secon-darily cleft for a short time.

Hammarberg has worked out the development of the frontalin great detail. He describes the stages in the canalization ofthe supra-orbital line, and the details of the secondary lamellaedeveloped from the main bone in this region. He goes on tosuggest that the frontal of L e p i d o s t e u s consists of threeelements fused. The present author cannot agree to this sug-gestion. The observations of Hammarberg have been con-firmed, but it does not seem reasonable to use the behaviourof secondary elements as evidence of the origin of the main bone.

It seems, therefore, that the 'frontaux principaux' are thefrontals and again that a compound name is not justified inL e p i d o s t e u s osseus .

OTHER ROOFING BONES.

The 'parietaux' follow the frontals. They were not observed

SKULL OF LEPIDOSTBUS 23

in the 25-mm. fish, but were far advanced at 38 mm. There isvery little to be said about them; the term 'parietal' is appliedto them by all authors, and is not in question-

In addition to the above-mentioned bones there is a trans-verse series related to the occipital commissure, and two pairsconnected with the shoulder girdle. These are all visible in theadult, but only those connected with the girdle were seen in thereconstructed specimens.

The 19-mm. fish had the cleithrum and supra-cleithrum welldeveloped, while the 38-mm. specimen had a small scalebonealso.

However, in the 11-cm. stage all the known elements werepresent in this region.

When the adult is studied, it becomes doubtful as to whethera post-temporal exists. Immediately posterior to the supra-temporal is the extra scapular which receives the lateral-linecanal from the supra-temporal, gives off the occipital commis-sure and finally passes the canal on to the supra-cleithrum;from the supra-cleithrum the canal passes to the first of thebody scales. There is a small scale-bone lying postero-mesialto the extra-scapular which contains a few pores of the latero-sensory system, but no portion of the main canal, and this isprobably a neomorph.

BONES RELATED TO THE INFBA-OEBITAL LINE.

It has already been stated that the ethmoid commissure couldnot satisfactorily be identified, but a few organs were noted inrelation to the antero-ventral pair of 'os mobiles du nez'.There is no objection to the term rostrals for these two bones,which are fused in the adult.

The lateral pair of ' os mobiles du nez' may be considered asbelonging to the same series as the elements of the infra-orbitals. Hammarberg's term antorbital seems quite sound.

Maxi l l a i r e s Supe r i eu r s .As has been suggested above, these are certainly not the

homologues of the maxillary of Amia or L e p i d o t u s . Theyare lateral-line bones and are developed as small gutters very

24 F. J. AUMONIEH

closely applied to individual organs of the infra-orbital line.In their earliest stage (25 mm.) those teeth which happen to benear to them have already made an attachment. As the bonesincrease in size more and more teeth become attached, tillfinally they assume the function of the maxillary: the maintooth-bearing element of the upper jaw. It will be noticed thatthe only portion of the latero-sensory system to be fullycanalized lies in this chain of bones. As has been mentionedabove, the most posterior of the 'os mobiles du nez' (ant-orbital) can be considered as a member of the series, though itmust be admitted that the canal does not extend on to it. Thetwo elements immediately posterior to this bone have fusedwith each other, but all the rest maintain their individuality.There is no evidence of separate tooth-bearing and lateral-linecomponents in the series, on the contrary, it is clear from thesections that they are true lateral-line bones which have ac-quired a secondary tooth-bearing function. (My friend Dr.Westoll has mentioned this fact in his work on Osteolepis.)These bones can be called the infra-orbital chain, on account oftheir relationship to the infra-orbital line. Further discussionmust be reserved till the next bone is dealt with.

' a , b , o.'It will be convenient to make a departure from the sys-

tematic treatment mentioned above, and to deal with this ele-ment here (though it is not a lateral-line bone).

Agassiz applied these letters to a series of three small bonesjust above the corner of the gape. He suggested that theymight be the 'maxillaires superieurs', while the chain of bonesto which he gave the latter name would then be called the 'inter-maxillaires'.

Allis, in a paper on the respiratory valves of P o l y p t e r u s ,mentioned this region of L e p i d o s t e u s . His words are: ' I tis, however, obvious that these bones (maxillary chain) ofL e p i d o s t e u s cannot be the homologues of the maxillarybones of A mi a and Te leos tomi , for, aside from the factsthat they have immovable relations to the skull, and aretraversed by a lateral canal, other and apparently true maxil-

SKULL OF IiEPIDOSTEUS 25

lary bones are found in what would seem to be their properplaces.'

This statement is in full agreement with my findings: thebones 'a, b, c' represent the true maxillary. Only one bonecould be found in the series of fish studied. It always lay ina fold of the skin ventral to the labial groove, while the lateral-line canal passed dorsally to the groove. As already noted, itbears a few teeth which are not functional in the adult. Thebone develops early, being first seen at 14 mm., while the infra-orbitals develop late.

It is surprising that Allis should abandon the true interpreta-tion in a later paper, yet this is so; for in his paper on the lateral-line bones of fishes he adopts the view that the infra-orbitalsare of compound origin, and though he is apparently notsatisfied with the term maxillary chain (which he puts in in-verted commas) yet he does not suggest an alternative.

Hammarberg in a recent paper has suggested the term'lachrymalia' for the infra-orbital chain. However, it is scarcelypossible to regard these bones as being a fragmentation of thesingle lacrimal of A mi a. They ought to be interpreted asarising from a multiplication of the organs of the infra-orbitalline due to the elongation of the snout.

It is possible that this condition could be explained as follows:The snout was being elongated comparatively rapidly and theneed for supplying a support for the teeth of the upper jawcould be met either by lengthening the maxillary or by extend-ing the circumorbital series. As the latter elements were lateral-line bones, they had already a series of focal points for bonedeposition extending far forwards, consequently it would havebeen easier for a small series of infra-orbitals to be developedin connexion with the infra-orbital line, rather than for themaxillary to be extended. If this were so, the maxillary wouldhave lost its function, and hence its degeneration is explained.

The bones labelled by Agassiz 'sous orbitaires' form anintegral part of the infra-orbital chain. They were not seenuntil the 11-cm. animal was examined, and it is probable thatthere is no clear division between the orbital and rostral por-tions of the chain until a late stage.

26 F. J. AUMONIEE

The next bones on the infra-orbital line are the ' mastoiidiens'.These receive the line from the circumorbitals and are thesupra-temporals. Pehrson uses the term 'inter-temporo-supra-temporal' for the corresponding bone in A m i a; out inLep idos t eus no evidence of a double origin could bedetected, and thus the compound name is not justified.

The bones related to the lateral line and which lie posteriorlyto the supra-temp oral, have already been dealt with.

BONES BELATED TO THE OPEBCULO-MANDIBULAR LINE.

The only bone of this series which has been reconstructed,and which is not a member of the lower jaw series, is the'preopercule'. This being the case it will be convenient todeal with the opercular series as a whole.

The 'preopercule' is clearly the preopercular. It carries theoperculo-mandibular line from the orbit till it curves dorsallyand joins the infra-orbital canal in the supra-temporal. Pos-terior to the preopercular lie the ' opercule' (dorsally) and the' sous-opercule' (ventrally). The terms opercular and sub-opereular for these two bones respectively have not beenquestioned.

Anterior and dorsal to the anterior ends of the preopercularare the bones called ' tympano-malleaux'. The term 'inter-opercular' has generally been applied to the bone in thisposition. Began, however, considers that it cannot be an inter-opercular as it is dorsal to the preopercular. He thereforeinterchanges the terms preopercular and interopercular, regard-less of the evidence of the lateral line, which he does not con-sider to be of sufficient weight. If, however, a comparisonbetween Lep idos t eus and L e p i d o t u s is made, it seemsmore likely that the interopercular has vanished and that thetympano-malleaux' are really the most anterior elements of

the series of sub-orbitals of L e p i d o t u s . This condition couldbe explained thus:

As the head becomes longer and narrower there is an increasein the space behind the orbit, this is filled by the posteriorelements of the sub-orbitals (bearing the horizontal pit-line)and by a new series of anamestic bones (the cheek plates). The

SKULL OF LEPIDOSTEUS 27

most anterior of the sub-orhitals, however, is squeezed betweenthe circumorbitals above and the preopercular below; as bothare lateral-line bones, they are more likely to survive than thesub-orbital, which is first reduced in size, and then cut offfrom its fellows.

Hammarberg calls this bone quadrato-jugal, chiefly becausehe has observed a fusion between it and a cartilage bone inthe quadrate region of the palatopterygoquadrate. In Lepi-dos teus osseus , however, no such fusion occurs even at11-cm. overall length.

PALATE.

The premaxillaries have already been discussed; the nextelements are the vomers. Goodrich considers that the termprevomer is better for the anterior palatal bones of fishes, sinceit is very doubtful whether the median vomer of mammals ishomologous with the paired bones of fishes.

The bones labelled 'palatins' by Agassiz are two quite dis-tinct structures; the anterior elements being the palatines, theposterior the ectopterygoids. Each of these elements is a singlebone from its earliest stage (14 mm.); there is, therefore, nosupport for Mayhew's contention that the ectopterygoid ofL e p i d o s t e u s consists of a fusion of palatine and pterygoidelements (palatopterygoid). That part of the bone called byAgassiz ' pterygoide' and which Mayhew calls mesopterygoid,is probably better called endopterygoid as is done by Began;Goodrich also seems to prefer this latter name.

The 'sphenoi'de principal' is the parasphenoid. There is nosign of a compound origin at any stage. In addition, there is noindication of a fusion with any chondrocranial ossification evenin the 11-cm. stage.

Lower J a w .The most obvious element of the lower jaw is the ' dentaire'.

This is the dentary but it is not a lateral-line bone in primitiveforms. In L e p i d o s t e u s , however, the ventral edge of thedentary is folded inwards ventrally to Meckel's cartilage, andacquires a secondary association Avith the lateral line. Here,again, there is an urgent mechanical necessity to account for

28 F. J. AUMONIBE

the precocious development of a non-lateral-line bone at theexpense of the lateral-line series, and in this case it should bementioned that the organs of the mandibular line are slowerin development than those of the other lines, while this lineis the last to become canalized.

With regard to the fusion between the lateral-line and non-lateral-line bones in the lower jaw (this is the only instanceof such a fusion), the union is of a temporary nature. It seemsprobable that in L e p i d o s t e u s p l a t y s t o m u s a dentaryis fused with a splenial, presplenial, and post-splenial; butin L e p i d o s t e u s ossetis a dentary is fused with a splenialwhile outgrowths of dentary invade the pre- and post-splenialmesenchyme condensations before independent bone-formationcan take place.

On the dorsal surface of Meckel's cartilage lie the 'comple-mentaires' or coronoids. There are two of these on each side, andthey show no features of particular interest. (In the 11-cm.fish the posterior coronoid of Mayhew was present.)

The 'angulaire' really consists of two elements. The larger,more dorsal element is a lateral-line bone, and is probably theangular; the smaller element becomes fused with a cartilagebone in the 11-cm. specimen and is thus the articular. Thisreverses the order of nomenclature adopted by Mayhew, whodoes not seem to have used the evidence supplied by thesensory canals in his conclusions on this region. The articularof L e p i d o s t e u s is of compound origin (dermal and endo-chondral).

On the lingual side of Meckel's cartilage and at the posteriorend lies the ' operculaire'. There can be little doubt that thisis the prearticular. Mayhew calls this bone splenial, but asa true splenial is a lateral-line bone and lies on the outer sideof Meckel's cartilage, there seems to be no justification for thisterm.

CONCLUSIONS.The most important question which must be answered in

connexion with the above work is: 'How far can the evidencesupplied by the canals of the latero-sensory system be trustedin establishing the homologies of the bones in which they lie ?'

SKULL OF LEPIDOSTEUS 29

It must be admitted that a final answer cannot be given here,but it may be permissible to suggest the following principles:

1. That lateral-line bones do not fuse with non-lateral-linebones, though they may fuse with fellow members of thesame series.

2. That lateral-line bones resist extinction or reduction moresuccessfully than non-lateral-line elements.

3. That a non-lateral-line bone seldom acquires a secondaryassociation with the lateral line, and this only happens underconditions of strong mechanical necessity.

In the above work no attempt has been made to emphasizethe closeness of the relationship between certain bones and thelateral canals, that would be superfluous; but sufficient evidencehas been gathered to show that the skull of L e p i d o s t e u scan be explained without recourse to the suggestion of fusionsbetween lateral-line, and non-lateral-line, bones.

The dentary appears to be an exception to (1) above; butin this case the following points suggest that the statement isstill generally valid. The lower jaw has the feeblest develop-ment of both cartilage and lateral-line organs of any part ofthe skull. Also the fusion of the dentary with the lateral-linebones does not seem to be permanent. That is to say in onespecies of Lep idos t eus three lateral-line bones become fusedwith the dentary, but in another species only one element isso fused.

From a careful study of the sections of all stages describedin this paper it seems perfectly clear that the expansion of thedentary does forestall the formation of the pre- and post-splenials and does not fuse with them.

Concerning the nature of the relationship between lateral-line canals and bones, no explanation can be offered. It maybe that the growth of the sense organs in the skin is initiatedby a factor which also stimulates the underlying mesenchymeto rapid growth and later to bone-formation; as, however, bonedoes not form under every organ, this is most improbable. Itis more likely that bone-formation is a balance between ancestryand mechanical needs. In the individual this would result inthe formation of bone whenever the mechanical stress is greatest.

30 F. J. AUMONIER

The young L e p i d o s t e u s is anchored by its nose, and there-fore has to stiffen its anterior end as swiftly as possible, hencebone is laid down as soon as the mesenchyme is in a position todo so. At a later date the pressure set up between the growingsense organs and the underlying structures leads to bonedeposition. This, however, is nothing more than a suggestion,as direct evidence is lacking.

The only remaining point of interest is that of the systematicposition of L e p i d o s t e u s . Since the classification of thegroup Holostei is at the present moment receiving attentionfrom Eayner and others it is only possible to suggest thatL e p i d o s t e u s should be placed close to L e p i d o t u s . Thiscan be supported by the following points of resemblance:

1. The premaxillaries have long posterior extensions.2. The circumorbitals are numerous and extend far forwards.3. There are numerous sub-orbitals, the most anterior of which

reach the articular while the rest are immediately dorsal to thepre-opercular.

SUMMARY.

1. The development of the dermal elements of the skull ofL e p i d o s t e u s osseus has been studied in a series of twelvestages.

2. The homologies of the three small scale-bones of the snouthave been established. They are the rostral, nasals, and ant-orbitals.

3. The ' maxillary chain' is shown to be an extension of theinfra-orbital series, and the existence of a true maxillary isestablished.

4. The extent and single origin of the premaxillary are shown,and its unusual association with the supra-orbital basal isdiscussed.

5. The correct homologies of the opercular series have beenestablished, the recognition of a sub-orbital has been recorded.

6. It is shown that Stensio's complex nomenclature is redun-dant to a description of the dermal bones of the skull of Lep i -dos teus osseus .

SKULL OF L E P I D O S T E U S 3 1

^REFERENCES.

Agassiz, L., 1833-43.—'Recherehes surles Poissons fossiles', 11, JJeuehateLAllis, B. P., jun., 1900.—"Premaxillary and Maxillary bones and the

Maxillary and Mandibular breathing valves of Polypterus biehir",'Anat. Anz.', 18.

1904.—"Latero-sensory canals and related bones in fishes", 'Intern.Monatssehrift f. Anat. u. Physiol.', 21.

Goodrich, E. S., 1930.—'Studies on the structure and development ofVertebrates.' London.

Gregory, W. K., 1933.—"Fish Skulls", 'Trans. American Phil. Soe.', 22.Hammarberg, Figge, 1937.—"Entwieklung des Schadels von Lepidosteus

platystomus", 'Acta Zoologica', 18.Mayhew, R. L., 1923-4.—"Skull of Lepidosteus platostomus", 'Journ.

of Morph.', 38.Parker, W. K., 1882.—"Development of skull of Lepidosteus osseus",

'Phil. Trans. Roy. Soe.', 173.Pehrson, T., 1922.—"Cranial Development of Teleost fishes", 'Acta

Zoologiea', 3.Regan, C. J., 1923.—"Skeleton of Lepidosteus with remarks on the origin

and evolution of the Lower JSTeopterygian Fishes", 'Proc. Zool. Soc'Veit, O., 1907.—"Besonderheiten am Primordialcranium von Lepidosteus

osseus", 'Anat.', 33.—— 1911.—"Entwieklung des Primordialcranium von Lepidosteus

osseus", ibid., 44.

EXPLANATION OF PLATES 1-8.

All figures are of L e p i d o s t e u s osseus . Left-side viewsare 'elevations', and dorsal views are 'plans', drawn as pro-jections without perspective.

The following symbols are employed on the figures:Dermal bones: fine line stipple.Cartilage: coarse dot stipple and fine dense dot stipple.Lateral-line organs: heavy black dots.

PLATE 1.

19-mm. stage.Fig. 1.—Left-side view showing dermal bones and lateral-line organs.Figs. 1 a, b, c.—Transverse sections at the stations indicated by the

lines a a, &c.Fig. 2.—Dorsal view showing dermal bones and lateral-line organs.Fig. 3.—Dorsal view; lower jaw alone, showing dermal bones.

32 F. J. AUMONIER

Kg. 4.—Left-side view showing palatine and ectopterygoid.Fig. 5.—Left-side view showing prevomer, ectopterygoid, and para-

sphenoid.

PLATE 2.

25-mm. stage.Fig. 6.—Left-side view showing dermal bones and lateral-line organs.Figs. 6 a, b, c, d, e, f, g.—Transverse sections at stations indicated by

lines a a, &c.

PLATE 3.

25-mm. stage.Figs. 6 h and i.—Transverse sections at stations indicated by lines

h h, &c.Fig. 7.—Left-side view showing palatopterygoquadrate, dermal bones,

and lateral-line organs.Fig. 8.—Dorsal view showing dermal bones of dorsal surface and organs

of supra-orbital line (on fish's right). Dermal bones of palate andorgans of infra-orbital line (on fish's left). The parasphenoid has beencut off at section d.

Fig. 9.—Dorsal view of anterior end (to section / ) showing palatal bones.Fig. 10.—Left-side view of anterior end (to section / ) showing dermal

bones of palate and lower jaw.

PLATE 4.

25-mm. stage.Fig. 11.—Dorsal view of lower jaw showing coronoids, Meckel's cartilages,

and quadrate region of palatopterygoquadrate (on fish's right); dentary,angular, articular, and sub-orbital (on fish's left).

38-mm. stage.Fig. 12.—Left-side view showing dermal bones and lateral-line organs.

PLATE 5.

38-mm. stage.Figs. 12 a, b, c, d, e, f, g, h, i, j , k.—Transverse sections at the stations

indicated by the lines a a, &c.

PLATE 6.

38-mm. stage.Fig. 13.—Left-side view showing dermal bones of infra-orbital series.Fig. 14.—Dorsal view showing dermal bones of dorsal surface and of

the infra-orbital series.Fig. 15.—Left-side view showing prevomer and parasphenoid.Fig. 16.—Left-side view showing palatine and ectopterygoid.

SKULL OF LEPIDOSTBUS 33

PLATE 7.

38-mm. stage.Fig. 17.—Dorsal view showing palatal series.Fig. 18.—Left-side view showing coronoids, maxillary, prearticular,

and angular.Fig. 19.—Dorsal view of lower jaw.Figs. 20 and 21 have been placed in the text as Text-figures 1 and 2.Fig. 22.—Dorsal view of chondrooraninm. A portion of the supra-orbital

bar on the fish's right has been removed to show the artioulation of thepalatopterygoquadrate with the basitrabecular process.

Fig. 23.—Left side of ehondrocranium.

PLATE 8.

38-mm. stage.Fig. 24.—Ventral view of ehondroeranium. The palatopterygoquadrate

and hyomandibula have been removed on the fish's right to show thecornu trabeeulae and basitrabecular process.

Fig. 25.—Dorsal view showing chondroeranium.Fig. 26.—Left-side view showing chondroeranium and dermal bones.Fig. 27.—Ventral view showing chondrooranium and dermal bones.

The anterior end of the eotopterygoid, palatopterygoquadrate, andhyomandibula have been removed on the fish's right.

NO. 329

a b

ParasphenoidJ4

FrontalSupra orbital line

Premaxillary-Maxillary

Labial groovecommissure

Palatal region ofMiddle pit line palatopterygoquadrate

MeckelVcartilage

a b

rostri,-Prevomer.-Palatine

1aDentary

Dentary'Angular

a b

Premaxil!arLj%

Operculo mandibuiar Horizontal pitline lineline

Septum rostri

Ectopterygoid --Maxillary —

Palatal region of

Dentary' yAngular

W) 1bM ,, „Meckel's

!, ., cartilaqe \ \Parasphenoid 3 Ectopterygoid>

1ca b

I I

Premaxi Mary - - 5-s^TT"

line

Roof of neurocranium a b,Supraorbital line

Palatal region; ' ofPdlatoptefygociuadrace

Angular

, -Middle pitline

- Occipitalcommissure

Ra ra sph enoid200400ji

Ectopterygoid Parasphenoid

Palatine

Frontal

a bParasphenoid

PrevomerEctopterycjoid

Dentary

a b

<7^

00o

QoO

sI

c d h i

.Frontal iupratemporalParasphenoid

Nasa l .

-ipNBianchiostegal ray

Dentary —

Premaxillary

^--Coronoid

h i

Septum rostriPremaxillary ',

Palatal region ofpalatopterygoquadrate

Septum rostri .,_ ! .Nasal

6a0 200 400 LL Meckel 's

cartilage

Septum rostriPrernaxillary ',Frontal v

Prernaxillary--

Palatine

o r 9 a n " Coronoid

6b Denta'ry Parasphenoid

--EctopterygoidL—Maxillary

"Prevomer

Meckel'scartilage

Dentary Meckel'scartilage

Palatal region of, - 'palatopterygoquadrate--Ectopterygoid

Olfactory nerve

Septum rostrij Olfactory lobe

Meckel's ,cartilage'^

Angular Parasp'henoid

6d

Quadrate region ofpalatopterygoquadrate ~ ~-

Dentary

Dentary

A n9u l ' a ' r Parasphenoid

6e

Ectopterygoid

-Vertical pit line•Meckel's cartilage

Neurocranium

Retina

Supraorbital line

Quadrate region of" "palatopterygoquadrate

Quadrate region of ^palatopterygoquadrate;/ P a r a s p h e n o j d

Angular gr

f

--""-"-Meckel'scartilage

Supraorbital

Frontal

Basicranialtrabecula

Ectopterygoid

_ _ Opticnerve

Quadrate region o f! palatopterygoquadrate

Parasphenoid

Sg

^-*fe

Oo

bo

Otic capsuleParasphenoid o 200 400

6h

'—Hyomandibula

Branchiostegal ray

Otic capsule

Preopercular

Parasphenoid

,-Supra temporal

- Branchiostegals ray

h i

Olfactory organ MaxillaryU ln>obital \

.Frontal ,.Supratemporal

Dentary A lar' ' • \rrerugoia region or N — * x — ^Coronoid m9uianArticular< Subdrbital *»palatopterygoquadrate X Preopercular p^ . ..

_ \ . Branchiosfceoal rau rarasPnenoia

/a

Olfactory organ

Supra orbital linePremaxJIIaryX

p Quadrate

Ic Id le |F

Frontal

Branchiosbegal ray

0 200400M

|h 1% _ Branchiostegal rayI^Viteplienoid

a

Prevomer

c d/Parasphenoid Prevomer wCoronoid Anqular.

Dentaru \felatine, \ l

o 200% / . T T r ^r 'Ectopcerygotd 'Maxillary

o*sS?

aCN3

fa Ib

Coronoidpeckers cartilage

Coronoid

0 200 400JUL

ab c d e

Premaxillary

ab c d

Articularf

^Suborbftaf

g h i

Frontal

J kSupratemporal

/Parietal ..-"' ,,-"Scalebone"

--Supracleithrum--Opercular

Oo

12 a

Rostral

Septum rostri! NOlfactory

organ '"

Nasal

Coronoid

if^O 1 2 e

Dentary 'cartilage *

Septum rostri

•, ^ v# v , ^- Infraorbi ta lEctopterygoid - - ^ ^ " ^ - - P a l a t i n e

Septum rostri •—•—>Premaxillary-. ; .Olfactory 0 400 800 JJL

12 fPrevomer \ $ ^ 4 ~ = ^ u

;/ . DentaryMeckel's Parasphenoidcartilage

Infraorbitalcanal

Premaxillary^Infraorbital

Septum rostriPremaxillary v l _ , F r o n t a l 1 Z 9

Palatal region o f .... P 5l t t d t e ~

Prevomer

p y g qEctopterygoid ̂

Palatine

Oentary

. . Septum rostriFrontal < \

Quadrate region o f ,.palatopterygoquadrate

PrevomerD e n t a r y ' / > i l ^ ^ t ^ " Meckel's

Angular' Parasphenoid cartilage

Roof of neurocraniumP a r i e t a I • ,Frontal

Ectopterygoid /Prearticular Parasphenoid

12 h

Palatal region ofpalatopterygoquadrate

-Dentaryx "Angular

Articular . , .Suborbital- - - — • f |

Ectopterygoi'd Parasphenoid

0 400 800

. Basicranialtrabecula

,-.NPterygoid region ofvv palatopterygoquadrate

Preopercular

FrontaJ Parasphenoid

Suborbital

Parietal

SupratemporaJ

Posterior taeniatecta media

121Basicranialtrabecula "

/.^Quadrate region of^ palatopterygoquadrate

Ectopterygoid

FrontalySupraorbital bar

Pterygoid region ofi "palatopterygoquadrate

~—Ectopteruqofd— ^_^ -, vHyomandibufa

Preopercular Paras'phenoid 1 2 k

irSi

oo

ab c d e PI f

I Organ of infraorbital linei \Pore of inftaoitital line

fenapsed portion of \ '. infraorbital ^ ^inrraoroital line x ! • —

hi j k

^Maxillary

I L

130 400 800JJL

i ^ CRostral y ^ a s a f ^

Anforbibai; j Anterior nostril'• Olfactory organ

Maxillary .Premaxillary Supratemporal

Nasal N\ AntorbitalAnterior nostril

infraorbital\\ \ \ \a o c A. e

14Scalebone*

d e/ I

Prevomer

g

9

Frontal Parietal' Supracleithrurh-H 'i

h i j

//Rarasphenoid

0 400 800|x

Si

2O

d ei i

h

h- PtevomerEctopterygoid

I SJ.

Preopercular

Palatine 17 Suborbital

ii ik

Coronoid

9 h '

Coronoid ,Maxillary Dentary

* —— -

(

BranchiostegaI rayq h

IP I IAngular Articular

18

a b c d <z 9n i i i j

Palatal region of\ palatopterygoc|uadrate

^ _ ^ _ _ ^ _ _ Coronoid ,Anqularv

n \ , '3 Particular Coronoid'' 19

Id fe

J

D \. , A

PrearhcularIf Ig Ih

Ih ij i | j Basitrabecular process.Quadrate region of /,Basicranial trabecula•Dalatooteruoofluadmte// j^teriorbenia tecta media

region ofpterygoc^uadrate

22

\ Cornu brabeculae

Septum rostri \Anterior fontanelle

Posterior fontaneile

Supraorbital bar - - .taenia

tecfca media

23s

x - > - v '"Basitrabecular process

D M region cF pala^rygoquadrate

ab c d e

Oo

<?*»»i

^

a b c d e F g h i j k ,

Quadrate region of palatopterygoquadrateI Palatal region of1- palatoDteruooauadrate

Pterygold region of^- palatopterygoquadrate

Posterior taentatecta media

Septumrostri

cornu M t c u t a Supraorbital bar

Premaxilla

' , Basicranial" t rabecula

•BasitrabecularPosterior taenia tecta media Pr o c e s s

Palatopterygocfuadrate /supratemporal /Scalebone

Nasal

Cornu trabeculaeSupra

^cleithrum

H I M Ia b o dl e. F

a b c d c

/! I l l

Frontal Anteri(/r f o n taneI leXa r s e t a l Posterior fontanelle25 I I I I Basrcrania/ trabeculs

j

h \ Basicranialb l

PremaxillaNasal

I IS I I trabeculaParasphenoid / ,

Palatal region of Frontal / / / ^Supratemporalpalatopterygoquadrate

Sepfcumrost r i -

Kostral ys

Prevdmer! Suborbital

\s^Subopercular

26

Prevomer

Kostral

Ectopterygoid . .; Endopterygoid \ \ 'Preopercular

Quadrate region of \Basitrabecular processpalatopterygoquadrate Pterygoid region o f

palatopterygoquadrateEctopterygoid | _ ^ — ^ _ _ ___

- -OpercularHijomandibula

Sepbum rostri

P a|a t fn e

27Parasphenoid | Endopterygoid \freopercular \Subopercular

Suborbital Basitrabecular process

ab c d e 9 h i j k

8

Oo