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Show !l26 ON TilE STRUCTURE OF THE SJ{UTJL. but it involves that of certai.n oth er bones of the side walls of tho skull. . Lizards the lateral walls of In the Chelonia, and Ill many. d 'the prefrontals aro b t en the pro-otws an ' the . crl aniumcu, pi.e ed wbe y cart 'l . or by n1embrane. In the dry I age, . entire y oc (F' . 88 C) it is true that there IS an ap-skull of the Turtl~ 1fJg. t 'f t11e pro-otic but this is only a b ll 1n ron o ' parent onyt dw aw n from the pan·· etal which becomes connected c ' pr.o che stls sente ryg°O i.d an d WI' tl small distinct lamella of bone. l a ' d wit 1e ~d'. t' t rod-like bone (Co, Fig. 90, A, an In Lizards a IS Inc ' . 1 f b . . cu ies a corresponding position, articu a Jng a _ovo Fig. 91), oc . p d b l rith the pterygoid, and recmves with the panetal, an e ow "' the name of the columella. Fig. 91. . d I ) B S the anterior prolongation F1.(J' 91 -The inter-orbital septum of a ~Jzar. ( guda.na • 'fi,..., t'ion. b the superior paired o• · . h · d the mfenor me mn oss1 ..._... , ' or beak of the bas1-s~ e_nm f; al, . t . ·b'tal septum. Co the columella. ossification of the lett Side o t 1 e m er-oJ 1 ' ' I b th Ohelonians and Lizards the basi-uranial ~xis IS laternal lyo e ompressed I. n the presph enol' dal reo-ion and IS con- b . 'l P'l In t d into a vertical inter-orbital septum, as Ill t 1e I re. ver eC helonians neither the septurn, nor tll e n1 embra.n ous' or ~~laginous, ~isphenoidal and_ orb_ito-spheno~dal :;o~~:::s: nected with it, present any ossifications,. but,_ In m_ y In the . 1 ·nre of bone are developed In this regwn. . dehcate a1n1 1 (F' . 91) the inter··orbital Iguana tuberculata, for examp e . I g. . d beak of the basi-septum is supported below by the prolonge . . s benoidal 1 'd Above this it presents a long median pre p d sp 1eno1 . ' :D k connecte ossification (a) forked posteriorly. Th? or s ar~ represent with two bones, one on eac~ side (b), whiCh appear or hi to-sphenoids. 1'HE SKULLS OF REPTILIA AND AVES. 227 The Crocodiles, on the other hand, possess a large and distinct lateral ossification in front of each pro-otic (AS, Fig. 88, B). This ossification bounds the foramen for the third division of the fifth nerve in front, and unites with the basisphenoid below and with the parietal above, so far agreeing with the alisphenoid. Since it extends so much further forward than the alisphenoid ordinarily does, Cuvier has suggested that it probably represents both the ali- and the orbitosphenoids; but Stannius has pointed out the existence of two small ossifications close to the optic foramina, with an insignificant descending median stem at their bases. The former he considers to be the orbito-sphenoids and the latter the presphenoid. In these Reptiles, in the Lacertilia and in the Chelonia, the basis cranii, as has been already stated, is modified anteriorly into an inter-orbital septum, as in the Pike ; but in the Ophidia, the Cyprinoid, or Batrachian, type of skull reappears, and the cavity of the cranium is continued without any sudden narrowing, from above downwards, from its posterior to its anterior end. In the Ophidian skull (Fig. 90, B) the side walls of the cranium, in front of the pro-otics, are completely closed in by bones, which might readily be taken for alisphenoids and orbito-sphenoids; but, according to Rathke, they are merely downward growths of the parietals and frontals, and therefore can have nothing to do with the true lateral cranial elements. The anterior part of the basis cranii in Birds is always vertically elongated into an inter-orbital septum, as in the Croco . dilia, Lacertilia, and Chelonia. In the Ostrich (Fig. 88, A) the presphenoid is completely ossified, but, in other members of 'the class, the nature and extent of the prosphenoidal ossifications may vary greatly. The alisphenoid is always well ossified, and occupies its characteristic position in front of the pro-otic and of the exit of the third division of the trigmninal nerve (Fig. 88, A). The orbito .. sphenoids, on the other hand, may or may not be represented by bone. In the Ostrich they are present, and are continuous with the presphenoid. Reptiles possess prefrontal and post-frontal bones, which usually remain distinct throughout life, and are admitted to Q 2 |