OCR Text |
Show 1867.] DR. J. MURIE ON PHASCOLOMYS PLATYRHINUS. 805 the four neural spines in advance of it are subequal in length, besides being shorter. Dorsal vertebrce.-Among the Marsupialia the dorsal vertebrae are thirteen in number, except in Phascolomys wombat, which has fifteen. P. platyrhinus, then, agrees with the latter in this respect; but P. latifrons, on the other hand, disagrees with its two allied specific forms, and reverts to the usual marsupial character. In the three species of Phascolomys the number of dorsal and lumbar vertebrae taken collectively are nineteen. Of these, in P. platyrhinus and P. wombat, as the preceding formula shows, there are fifteen dorsal and four lumbar ; but in P. latifrons there are only thirteen rib-bearing dorsal, and consequently six lumbar vertebrae. The bodies of the dorsal vertebrae in P. platyrhinus and P. wombat resemble each other, excepting in size. The under surfaces of the centra are slightly laterally compressed, which gives the appearance of increased vertical depth, which they otherwise do not possess ; this is most marked in the large adult male of P. platyrhinus. The bodies increase in antero-posterior thickness from the first to the last. In the size of the bodies of the dorsal vertebrae P. latifrons agrees most with P. wombat. One specimen of P. latifrons presented a peculiar flattening of the under surface of the bodies. In the largest P. platyrhinus the laminar arches are flatter and altogether broader. A better comparison is made between P. wombat and P. latifrons, on account of the equality of size; this brings out the fact that in the latter the neural laminae, as seen from above, are relatively narrower than in the former species. In one specimen of P. latifrons the neural spine of the first dorsal possessed a bifid tip, in all the other specimens of this and the two species compared it was single. In P. platyrhinus the same spine at its upper half has considerable antero-posterior flattening. This terminal flattening is just observable in P. wombat; but in P. latifrons, excepting the divergence of the bifid extremity, the spine is laterally compressed. The remainder of the neural spines are relatively longer and more laterally compressed in the species P. latifrons; their antero-posterior diameter is also greatest. In the powerful body of P. platyrhinus the dorsal spinous processes have each a bulbous extremity, and the sides of each spine are marked by grooves for the attachment of muscles. All three species have the first dorsal spine the longest, those behind diminishing gradually until they reach the shorter and broader neurapophyses of the lumbar region. In the Common Wombat, as Owen has mentioned*, the metapophysis rises suddenly from the outside of the prozygapophysis of the twelfth dorsal, increases in length to the second lumbar, diminishes by degrees to the second sacral, and is rudimental in the following sacral and caudal vertebrae. The same remarks apply to the Platyrhine Wombat. * Osteological Catalogue Coll. Surg. vol. i. p. 330 (1853). |