OCR Text |
Show 262 MR. ST. GEORGE MIVART ON THE [Apr. 28, girdles, and by means of membrane. These lateral structures constitute paraxial parts. These paraxial parts, unlike the neural arches, are not only always incomplete below, but they are not usually ossified continuously with the centra respectively supporting them, but generally consist of two parts-a transverse process and a rib-the rib being somewhat moveably articulated to the distal end of the transverse process. The ribs end freely, except those articulated with the pelvic bones, which are intercalated parts of the appendicular skeleton. Certain vertebrae have transverse processes only, their free ends more or less widely diverging. This is the case with the caudal vertebrae, except the first two or three of them (which sometimes support ribs), and also occurs in many trunk-vertebrae of Amphiuma, Proteus, and Siren. The caudal vertebrae, except the first one or two and the very last, have almost always inferiorly extending processes and arches, continuously ossified like the neural arches above them, which moreover they resemble, more or less closely, in shape and proportions. They are probably serial homologues of subcentral processes of the trunk-vertebrae ; and the whole of these inferior structures belong to a system oi hypaxial parts*, and are the hypapophyses. That such subcentral processes really do answer serially to the caudal hypapophyses behind them, is well shown in Siren, where the vertebra in front of that which bears large hypapophyses is furnished with a pair of small backwardly projecting processes exactly like those of Spelerpes rubra, but at the same time developed from the posterior end of the ridges, which unquestionably represent the hypapophyses of the vertebrae next behind (fig. 14). Sometimes instead of, or besides, lateral processes, the inferior surface of a centrum will develope a median longitudinal bony ridge. Such a structure is to be seen in the third and fourth vertebrae of Siren (fig. 13, Hy), the second and third vertebrae oi Menobranchus, and in many of the trunk-vertebrae of Proteus and Amphiuma. This ridge is hypaxial, and may also be spoken of as hypapophysial-f, as sometimes in Siren and Menobranchus it seems, by becoming medianly grooved, to divide into a pair of hypapophyses. The propriety of regarding the subcaudal arches and processes as distinct from paraxial parts is justified by the frequent coexistence of the latter together with subcaudal arches in the tail. Moreover these arches are undoubtedly the representatives of the "chevron bones" of the Crocodile ; and these latter were found by Professor GoodsirJ to be, at the root of the tail, enclosed within the backwardly continued peritoneal folds and abdominal cavity, thus removing them altogether from the * As before said, I believe the hyobranchial apparatus, jaws and trabecular, also belong to the system of hypaxial parts. t The way in which the osseous extension (of the under surface of the centrum) related to the great arterial channels is represented by a single process, by a pair of processes, or by a triple development, is well shown by Professor Owen (Memoir on the Megatherium, Phil. Trans. 1851, part 2. plate Iii. figs. 48-51). J Edinb. New Phil. Journal, January 1857, p. 128. |