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Show 202 LECTURE XI. ON THE STRUCTURE OF THE Sl{ULL. THE SKULLS OF FISHES AND AMPHIBIA. 0 The crant.u m, const.s tt'n g chieJlUl y of cartilage and without · ·a eradded membrane bones. cartilage bones, but wt ~ supd t Ganoids the Sturgeons, and The skulls of the chon ros eous ' . t . . his t e of structure, which forms a mos Spatula~ue exem?~Ify tl l' {pbetween the skull of Plagiostomes interesting transitwna In d h kull of ordinary osseous fishes. . an t e s l ria has a completely cartilaginous skull, produced In S:p_atu a . flattened from above downwards. The front Into a great beak, t' f at fewest seven of the anterior cartilaginous representa Ives o ' ' Fig. 81. . . .· . asc) and posterior (p~c) .Fi 81 -Side view of the skull of Spatulana With t;.~ ~nt~~~~~er . Or, the orbi~ WJ tl:. g. ve~·tical semicircular canals exposed.-Au~dthe au J ~~t~·ls . Br· the representati ves o the eye; N, the nasa l sac; J-fy , the hyo1 can app:u a ~ ' ' 1 . Mn the mandible, the branchiostegal rays; Op, the opcrcu urn ' ' THE SKULLS OF FISHER AND AMPHIBIA. 203 vertebrro of the spinal column coalesce into one mass with on another and with the skull. The notochord, extremely large in the spinal column, rapidly diminishes in size as it enters the skull, and, becoming a mere thread, terminates behind the pituitary fossa. The auditory organs a.re contained in large postero-lateral projections of the cranial mass, with the outer sides of which the suspensoria are connected. The base of the skull is protected by a long parasphenoid, which extends back under the anterior part of the spinal column ; in the dorsal region it presents an anterior and a posterior pair of perichondrial ossifications, separated by oblong laminre from lateral bony plates of the same character, but the homology of these bones with those in the roof of the Teleostean skull is not, to my mind, satisfactorily made out.* The suspensoria! apparatus of Spatularia consists of a single bone (A), compressed from above downwards superiorly, and from side to side inferiorly, with a superior and an inferior cartilaginous epiphysis ; to the lower cartilaginous epiphysis the operculum ( Op) is attached, and a short thick prismatic cartilage (B) is united by ligament with, and can play freely upon, its anterior and inferior angle. Posteriorly the lower end of this cartilage (B) is connected by ligament with the hyoidean arch (Hy), which consists of two portionA on each side; a small upper piece, with which the flat bone (Br), representing a branchiostegal ray, is connected; and a long lower ramus, the middle third of which is bony, while the two ends are cartilaginous. Anteriorly, the lower end of the inferior suspensoria! cartilage (B) is united by ligaments to two cartilaginous semi-arches (D and Mn), of which the upper (D) is articulated by a transversely convex head with a concavity of the lower (Mn ). The upper semi-arch is ligamentously united to its fellow in the middle line, and is suspended by ligamentous fibres to the under part of the prefrontal region of the skull. .A_ long flat bone (E), the hinder end of which is cut off in the specimen figured, lies on the outer side of the cartilage (D), and extends to the middle line. A second long flat bone is closely applied to the inner * See "Spatulariarum Anatomiam clcscripsit Tabuluque illustnwit Albertus Wagner." Berolini, 1848. |