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Show 804 DR. J. MURIE ON PHASCOLOMYS PLATYRHINUS. [June 27, Cervical Vertebrce.-It has been remarked by Prof. Owen* that in the Koala and Wombat the body of the atlas remains permanently cartilaginous. This observation appears not to be exceptional in the case of Phascolomys platyrhinus and P. latifrons, where the bony ring is incomplete in both. In the specimens examined by m e of P. platyrhinus the bony separation was as much as 0*4 of an inch, and in P. latifrons, as in P. wombat, about 0*3". The first cervical vertebra in P. platyrhinus, besides slightly larger dimensions, has a sensibly higher arched lamina than either P. wombat or P. latifrons, and there is the rudiment of a neural spine. The breadth from before backwards of the neural arch is usually greatest in P. latifrons; but this is not constant. The transverse processes seem also relatively longer and broader in P. platyrhinus, although this may only appear to be so from the generally increased size of the vertebrae. They, however, are flatter in P. latifrons, in this respect approaching to the shape found in the Koala. The greatest diameter to the tips of the transverse processes in the specimens compared respectively measured 2"*6 in P. platyrhinus and 2"*2 in P. wombat and P. latifrons. The antero-posterior diameter of the body of the axis in P. latifrons is comparatively greater than in either P. platyrhinus or P. wombat. In P. platyrhinus, however, the perforated transverse processes extend considerably beyond the body, whereas they are short in P. wombat and P. latifrons, where they barely reach outside the anterior articulating surfaces. The peculiar feature of the neural spine of the axis in P. latifrons is its possessing only in a slight degree the anterior projection, which is strongly marked in P. platyrhinus, and even relatively more so in P. wombat. The superior border from this to the summit of the spine is also very perpendicularly inclined in its arch in P. latifrons ; and both behind and before the neural spine and laminae there are not such deep concavities as in the two other species. In this manner the neuial spine has a sharper and half-crescentic form, distinguishing it from the other species. The two specimens of P. platyrhinus examined varied, inasmuch as one had a spine of the shape attributed to P. latifrons ; the other reverted to what is found in P. wombat-namely, where the spine is derived from broad laminae whose anterior and posterior edges are deeply concave. In the succeeding cervical vertebrae P. wombat has the neural spine of the fourth slightly longer than those of the third, fifth, sixth, and seventh, which last are all about equal in length. In P. platyrhinus the third cervical has only the rudiment of a spine, which is bifid and corresponds to a tubercle on the posterior arched concavity *of the neural spine of the axis. The four cervical vertebrae posterior to the third have their spines nearly alike in length; the seventh, however, is perceptibly the longest. In P. latifrons the seventh neural spine is by far the longest (with the exception of that of the axis); * Op. cit. p. 394. |
