OCR Text |
Show 1870.] AXIAL SKELETON OF THE URODELA. 269 open in front, is thus bounded internally by the centrum, superiorly by the tubercular process, and inferiorly by the capitular one. Ficv Q TJnderview of four trunk-vertebra? of Siren (No. 5 7 6 B in Museum of College of Surgeons). c. Capitular process. Hy. Hypapophysis. The distal end of each trunk- transverse process is provided with two superimposed articular surfaces for corresponding parts of each rib; these surfaces may be in close apposition, as in Cryptobranchus and others, or they may be separated by the before-described distal tendency to bifurcation of the transverse process as in Menobranchus, Amblystoma, and others. The only exceptions are offered by many of the trunk-vertebrae of Siren, Proteus, and Amphiuma, in which forms only the more anterior trunk-vertebrae support ribs. The transverse process of the last trunk-vertebra (which is connected indirectly with the pelvic girdle) is generally stouter than the others. In the caudal region this part generally undergoes a marked change, though sometimes, as in Cryptobranchus and Menopoma, the transverse processes of the caudal vertebrae remain much the same (except successively decreasing in size) as their anterior homotypes- the first two, at least, having still the double distal articular surface. In most cases, however, as in Menobranchus, the process becomes more pointed at its free extremity, and generally, in the anterior caudals, projects more or less directly outwards and but little backwards also, unlike the more anterior transverse processes. Sometimes, as in Siren (fig. 8), the tubercular part of the transverse process becomes entirely suppressed at the second, third, or fourth caudal vertebra provided with large hypapophyses, while the capitular part remains traceable for a variable distance further back along the tail, even to the last vertebra but thirteen in Siren*. In Cryptobranchus every trace of a transverse process disappears at the ninth or tenth caudal vertebra, if not earlier, and at the seventh or eighth in Menopoma, and sixth or seventh in Menobranchus. Sometimes, as in Salamandra, traces of the transverse process exist almost to the end of the tail. Occasionally, as in Triton palmatus and Pleurodeles waltlii, and others, the caudal transverse processes soon cease to project much outwards, but are to be distinguished for a very long distance as ridges connecting the interzygapophysial ridge above with the * E.g. in Brit. Mus. specimen. PROC. ZOOL. Soc-1870, No. XIX. |