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Show 810 DK. J. MURIE ON PHASCOLOMYS PLATYRHINUS. [Julie 27, lous among the Marsupials, by reason of its having fifteen ribs, in contradistinction to the other genera of its family, which have thirteen, excepting the Petaurists, where they are twelve in number. The Phascolomys latifrons, however, reduces this anomalous character to specific variety, for in it we have a return to the usual Marsupial number of thirteen. This diminution in the number of ribs may of itself be regarded as a marked character, fully justifying and carrying out Prof. Owen's proof, from the skull, of the specific distinctnness of the animal. This circumstance, however, is unfortunate for the Professor's reasoning upon the adaptive design of Wombats having fifteen pairs, as opposed to other genera of the Marsupiata. He remarks*, "The pressure to which the trunk of the Wombat must occasionally be subjected, in its subterranean burrowings, is probably the condition of the development of the additional pairs of ribs in that species." But in P. latifrons we have a doubtless burrowing Wombat wherein no more than the usual development of ribs in the Marsupials is found ; for Mr. G. F. Angas's observations**!- lead to the belief that, so far as scratching and burrowing are concerned, the Hairy-nosed Wombat is a thorough adept in the art. The ribs in the Platyrhine Wombat bear a proportion to its much more strongly built body; and anteriorly the grooves for the attachment of the intercostal muscles are well marked. Both the Common and Hairy-nosed species have rounder as well as weaker costae. In all the species the first rib is the flattest and shortest one. Bones of the Fore Limb. Scapula.-There is a distinct and well-marked difference in the form of this bone in the three species of Wombat, and such as would not occur by mere growth from a younger towards an older condition- that is, supposing that P. wombat were but an immature stage of the larger-sized P. platyrhinus. For example, the proportional breadth to length is in P. platyrhinus as 72 to 100, in P. wombat 53 to 100, and in P. latifrons 56 to 100, the preponderance of breadth to length, then, being greatly in favour of the Platyrhine species. In P. platyrhinus the scapula approaches to a square form, and is not oblong as in P. wombat, or irregularly oblong as in P. latifrons. The total length of the bone is more nearly alike in the three species ; but the relative breadth in the first named is almost one-third greater. The entire bone in P. platyrhinus is immensely strong and massive, and the free margin of the spine is unusually broad for attachment of the muscles. In P. latifrons this bone does not conform to the remarkably regular oblong quadrate figure which Prof. Owen has pointed out to be characteristic of P. wombat *};, where the superior and inferior costae run nearly parallel the one to the other. * Trans. Zool. Soc. vol. ii. p. 396. f P. Z. S. 1861, p. 2/0. $ Trans. Zool. Soc. vol. ii p. 399. |
