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Show 272 MR. CT. GEORGE MIVART ON THE [Apr. 28, but nevertheless show indications of separation exactly corresponding to the distal ends of the transverse processes to which they are attached, having two superimposed articular surfaces when the last-mentioned processes have such, and presenting a longitudinal groove in front and hehir.d each rib, the two grooves being most marked at the proximal end of the rib, and vanishing distally. The number of trunk ribs which thus bifurcate proximally is sometimes as many as eighteen* on each side; but the number is subject to some variation even in the same species. Thus in some individuals of Salamandra maculata (fig. 12) I have found three ribs (on each side) thus bifurcating, in others six ; and in Triton cristatus twelve pairs of ribs will sometimes be thus conditioned. It is always, however, towards the anterior and posterior ends of the trunk that this bifurcation tends to disappear, and mainly towards its posterior end, the bifurcation generally commencing at the second vertebra when the first one has its proximal end undivided. The sacral rib rarely bifurcates^, and it generally differs from the preceding ones by its greater stoutness, though it is usually short. The ribs in general maintain a tolerable equality of length throughout the trunk ; very rarely (only in Taricha) the ribs increase markedly in length towards the middle of the body, and then as obviously become again shorter. In Axolotl the second, third, or fourth is the longest pair, and thence backwards they markedly decrease in length to the sacral, which is suddenly much longer again. Occasionally they are very short indeed, being much less in length than the vertebrae to which they are attached ; this is the case especially in Proteus, Amphiuma, and Siren, where they are rudimentary (the posterior ones being quite so) and, as has been said, iew in number. In Spelerpes, Amblystoma, and especially Plethodon, they are very short indeed, never, except very slightly, exceeding a single vertebra in length. Generally they do not equal in length two midtrunk-ver-tebrae of the same individual; but Pleurodeles differs from all the other Urode/a in the length and strength of its ribs, the longer ones considerably exceeding the length of two of the longest vertebrae of the body. As has been said already, ribs are not unfrequently developed behind the sacrum. I have seen two such pairs in Glosso/iga poireti, and traces of such in Plethodon and Amblystoma ; one at least is sometimes present in Menobranchus, and often two and sometimes three (if not more) pairs are present at the root of the tail in Menopoma and Cryptobranchus. These postsacral ribs, however, never bifurcate at their proximal ends, and they are always short and more or less rudimentary. The first rib of all is generally stouter as well as shorter than the ribs of the trunk which follow after (fig. 11). Not unfrequently it developes a process from its outer or upper side towards its distal end ; this process projects outwards and somewhat upwards, and very rarely is so considerable as almost to equal in size the remainder of * Spelerpes rubra. British Museum. t It does so in Spelerpes rubra and Amblystoma punctatum in Brit. Mus. |