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Show 224 ON TIIE STRUCTURE OF THE SJ\UIJTJ. a separate centre, which subsequently ~oa~esces with ~he snpr~occipital; so that, just as the opisthotiC In. tl~ose animals orchnarily coalesces with the ex-occipital, the ep1otw anchyloses with the supra-occipjtal. In n1any Reptiles, though two of the three periotic bones coalesce with their neighbours, the suture between the throe persists on the inner surface of the skull, and is always shaped like a Y (Fjg. 88, B); the stem of theY answering to that part of the suture which separates the pro-otic from the opisthotic ossifications, while the diverging branches of the Y correspond with the suture between the opiotic and pro-otic in front, and that between the epiotic and opisthotic behind. In tho Turtle an interspace filled with cartilage takes the place of the stem of theY (E'ig. 88, 0). In the adult Crocodile the epiotic is united with the supra-occipital, and the opisthotic with the ex-occipital ; but that process of the opisthotic (c, Fig. 89, A) which separates the fenestra oval is from the fenestra rotunda (the anterior and inner edge of which, only, is completed by bone) where it meets tho pro-otic below and anteriorly (at d, Fig. 89, A), sends downwards and backwards a process, which curves round the cochlea, and, expanding to a broad plate, adjusts itself by harmonia (at b) to the outer and lower edge of the opisthotic, and to part of the posterior edge of the pro-otic. The anterior and inferior angle of the broad plate is thicker than the rest, and is seen in tho interior of the dry skull, at the bottom of the stem of the Yshaped suture (*, Fig. 88, B). If, as has been remarked, this part of the curved coehlear plate of the opisthotic be pressed cochlea rests jn the fo sa a, formed by the basi-sphenoid and basi-occipital. The upper end, bounded externally only by cat tilage, has disappeared jn the dry skull aJlll, with it, the outer lip of the f enestra rotunda, the plane of which is hori zontal, and nearly on. the level of the dotted line leading from Op. 0 in the fig ure. d is a small process of the pro-otic, against which the bend of the curYed cochlear pr ocess (b) rests. The dotted line from b indi cates the position of the suture between the hinder end of that process and the remainder of th e opisthotic bone. Ca, t he carotiJ canal; En, th.e upper opening of the posterior of the two canals by whieh each tympanum communic ·ates wi th the common Eustachian tube. The narrow anterior tympanic canal opens just in front of Ca, the cleft-like aperture being traversed by the dotted line from d. ln the Turtle's skull (B) Op. 0 is a distinct bone from E . 0, and sends dow~1 a pr?ccss between f.o., the f enestm ovalis, and f .?·., the f enestra rotunda, which term mates. 1n no recurrent hook, but otherwise corresponds exactly with t he cochleAr process (c) Jn the Crocodile. 1'IIE SKULLS OF REPTILIA AND AVES. 225 upon with a point, it gives ~ay; but this is not because it is merely suturally connected With the periotic bones, as has been suppo~ed, but because .the l~mina of bone by which the cochlear plate Is fixed to the opisthotiC is very thin and elastic. of ~n1on{ the many singular speculations which the historian e t ~ory of t~e s~cull will have to record' erha s the ts trabn gestd . Is . that whwh Identifies this coc1 1 l p . p . 1 oar oop, Imagined o e a .IStinct bone, with the entire ,, petrosal" bone f tb Mammaha. o e I~~irds, .the three periotic bones anchylose with one another as wei t als Wl~h the adjacent supra-occipital and ex-occipital s~ (;:~ ;s: ~/ at even theY-shaped suture becomes obliter~ted . t:hek d~Jermination of the homologues of the periotic bones ~n e. s ~ s of Birds and Reptiles, and more especially of the pro-otic, Is not only a matter of first-rate importance in itself, Fig. ~ 0 . A B 0 FiO', 90 -A a ·t· l 0 • ' vei Ica and lonO"itudi . 1 t' f similar section of the 1 ol1 f naS sec Ion o the skull of a Lizard (Cyclodus) B a of t h e Li. zard's skull· s* < *n osos ifia t n' ake . (PY t li1 ?n ) · t ' mem b ranous space in t•h e ro' of of the Lizard . Cm 'tJ ~- . cia wns m the Internasa l ca.rtilage ; Co the cohtmella s t apes. ' ' le uone a so called " col u me l~' a, " w h1' ch corresp'o nds with the Q |
