OCR Text |
Show 204 ON THE S'fRUC'l'UHE OF 'fHE SKULJJ. surface of the cartilage and follows its curves, from its hinder to its anterior extremity, overlapping and folding over the upper edge of the anterior three-fifths of the c~rtilage. Betwe~n tho hinder part of E, here cut away, and D, JS a space occupred by the levator muscle of the lower jaw. The mandibular cartilage extends to the syn1physis, and is coated externally, and partially embraced by, the flat bone (Mn ), the greater part of the upper edge of which bears teeth. On comparing these parts with those of the corresponding apparatus in the embryonic fish (Fig. 72), it becomes clear that the pieces A and B answer to the hyomandibular and symplectic, taken together. Indeed, at first sight, A, Rnpporting as it does the opercuhun, seen1s to answer to the hyomandibular, and B to the symplectic itself; but then it may be suggested that the hyoidean apparatus is attached at the distal end of B, and not between it and A, as it would be if the two corresponded, respectively, to the hyomandibular and symplec.tic. The cartilage D obviously answers to tho palato-quadrate arch, and that of the lower jaw to Meckel's cartilage. The fact that a levator muscle of the lower jaw passes between E and lJ seems to prove the former to correspond with a rnaxilla ; in which case the internal bone would be a sort of palatapterygoid, similar to that we shall meet with in Lepidosiren. The skull of the Sturgeon (Accipenser), like that of Spatularia, is greatly enlarged, posteriorly, by the coalescence with it, and with one another, of six or seven of the anterior vertebrro. In front, it is prolonged into a triangular snout or beak (a, Fjg. 82 ; a, Fig. 83), the wide base of which is formed by the antorbital, or prefrontal, prominences which separate the olfactory chambers from the orbits. Behind the latter are the two great projections ( c, Fig. 83) which contain the auditory organs ; and behind these again, and separated from them by a deep lateral fossa, are two wing-like processes (b, Fig. 82), which are directed outwards and obliquely backwards, and proceed, not from tho walls of the cranium proper, but from those of the spinal column, where it joins the skull. At this point there is, in the craniospinal cartilage of both the Sturgeon and the Spatularia, a great dilatation of the ·neural canal, which i::; clo::;ecl above only by a THE SKULLS OF l!'ISHES AND AMPHIBIA. 205 Jnmnbranous fontanelle. The skull . h ll T . P1 oper as no such fo t ne e. here JS a well-marked pitu 't . .!' n a-h d l . . I ary lOSsa, and the noto c or , ' vI eryb t nck In the spinal colum n, t aper,s to a thread as 't-enters t 1e ase of the skull and ends b h' d th' .!' 1 Th · ' e In IS 10ssa. e bones whrch are developed in r I· t. . . . ginous cranium in the base of the I ll e c~ IOn wrth tlus cartila-noid which extends back under til s ru I ared,-a gr~at parasphe-e coa esce anteno 't b and forwards to the level of the nasal ·r . r ver e rre, median bone in front of this whi h dcavlr.res' and a slender I · ( ' c un er res the rostral r ongatwn c), and appears to represent th . c p o- N d . t' . 0 VOlllel. o IS Jnct ossifications protect th I t 1 b e a era walls of th k II ut the. bones 1narked F (Fig· 82) sen d d own proc e s:D u ) short chstance, and the parasphenoid . ff esses or a longations upwards and outwards, from~::~ o. d tr~nsverse. proof its length) as in most fishes. SI e o tho Iniddle The roof of the skull presents a numbe . . ossifications, no one of whr'ch I.n vo I ves t h e s br · of dt istinc. t flat and which varv very much . t u ~acen cartilage , J In con our and t t · d. specimens. The general arran ement .c ex en I~ Ifferent sented by the . g Is, however, fan·ly repreaccompanyrng figure (Fig. 82). Fig. 82. Fig . 8(-) ·-Th e cart '1l aginous skull of St . sl.laded, and is supposrd to b a urgeon, With the eranial bone·. Tl fi . . l'l?ge formed by the s inous e l~een through the latter' which are left un:fl.l Ol mer Js WI I~ one ~nother· and ~ith th~ :{~~;:-' tf1 t~ '. ~n te_rior· ~·erle br·re, which ha v; !:~~ -;;,"J pos! IOU of the auditory orrraus. Na ' ''t<~ eta fwlng-hke processes; c, rostrum . A I:> ' ' post JOn o the nasal sacs . ' ''' 0~ these bones, the pairs C C both ru position and chant ' and .n' D clearly represent, of the Pike while F F . < ·~ter, the parwtal and frontal bones of that fish.' ' Simi arly correspond with the squamosals In position, agar·n , E-{ answers to the ethinoid, H, H to |