| OCR Text |
Show 268 MR. ST. GEORGI MIVART ON THE l_Apr« ^> greater number of the caudal vertebrae, and in the genera Proteus, Menobranchus, and Siren (fig. 18) in the cervical vertebra also. Generally the fully developed transverse processes are more or less cylindrical or somewhat compressed from before backwards, their long diameter, however, extending outwards and more or less (fig. 15, c) obliquely backwards, as in Menopoma, Cryptobranchus, Salamandra, & e Sometimes, however, as in all the transverse processes of Siren (fig. 9, c), except the first one or two, and as in the posterior ones oi Proteus and the middle ones oi Amphiuma (fig. 16), each one, though very short, is wide, i. e. much extended from before backwards. Each such transverse process springs generally from about the middle of each centrum's length, but sometimes, as in most posterior trunk-vertebrae of Siren (figs. 14 & 9) and Amphiuma (figs. 1 & 16), mainly from near the anterior end of each vertebra, and sometimes, as in the anterior trunk-vertebrae of Amphiuma, Menobranchus, and Siren (fig. 13), from near the hinder end of each. The backward inclination of the distal end of each transverse process is sometimes rather more marked at the anterior part of the series, as in Siren, Amphiuma, Menopoma, and Cryptobranchus. Each of these fully developed transverse processes is more or less excavated by a groove or depression on its anterior surface, and another one on its posterior face ; and these excavations are often such as to cause more or less of a division of the distal part of such transverse process into a superior portion and an inferior part. It is also to be observed that each transverse process at its point of origin from the vertebra, is connected below with the centrum of such vertebra, while above it is continuous with its neural arch and more or less distinctly continuous with the interzygapophysial ridge. Thus each transverse process may be regarded as made up of two parts, each ending distally in a more or less distinct process, the upper one of which is called the upper or tubercular process *, while the lower one is termed the lower or capitular one. Similarly it will be convenient to speak of the rib as made up of two portions, a superior tubercular part and an inferior or capitular one. When the transverse process is nearly cylindrical, as in Salamandra and most forms, these two component parts are of about equal antero-posterior extent; but sometimes, as in the middle trunk-vertebrae of Amphiuma (figs. 4 & 5), Proteus, and Siren (fig. 2), the capitular part is much wider from before backwards than is the tubercular part. The same is observable in a less degree in Menobranchus and Aneides. When (as in Siren after the first nine vertebrae) the tubercular process is also wide, and the groove before mentioned almost or entirely confined to the front surface, where it is immense, the transverse process assumes the form of two triangular plates (of which the upper is the smaller) united to the centrum by one margin and to each other by their two posterior edges, leaving a space between them which widens as they (the plates) diverge forwards (fig. 3, c. i). This space or fossa, * B y analogy with the condition of higher vertebrates in which the rib has a "tubercle" and a "head " respectively articulating with parts homologous with those here described. |
