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. 

Underview of four trunk-vertebre of Siren (No. 5762 in Museum of College 
of Surgeons). 
ce. 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-vertebrze of Siren, Proteus, and Amphiuma, in which 
forms only the more anterior trunk-vertebree 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 Oryptobranchus and Menopoma, the 
transverse processes of the caudal vertebra 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 back- 
wards also, unlike the more anterior transverse processes. Some- 
times, 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 Crypto- 
branchus 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. Zoou. Soc.—1870, No. XIX. 
