| OCR Text |
Show 684 MR. F. G. PARSONS ON THE [June 16, The Osseous System. Owen, in his ' Comparative Anatomy of Vertebrates,' has given an excellent account of the Kangaroo's bones, and I only intend to draw attention to certain points which seem to me of special interest. In the first place, the chief characteristics in which the skull of Petrogale differs from that, of Macropus are :- (1) In Macropus the nasal bones have an equal breadth in their anterior two-thirds, the posterior third being only slightly broader. In Petrogale the nasals are much more slender in comparison and their posterior half is considerably broader than the anterior. (2) In Macropus the fronto-nasal suture comes farther back than the most posterior point of the fronto-maxillary in most cases. In Petrogale the condition is reversed. (3) In Macropus the zygomatic process of the maxilla projects downwards below the cutting-edge of the penultimate molar. In Petrogale it is seldom much lower than the alveolar margin of tbe maxilla. I am not inclined to lay much stress on this distinction, since it seems that the processes of the Kangaroo's skull increase with age. (4) In m y specimen of Petrogale the infra-orbital canal was double on both sides, a condition I have found in two out of five skulls. In Macropus the canal is occasionally double on one side, but I have not found it so on both sides once in the thirteen skulls I have had the opportunity of examining. (5) In my specimen of Petrogale a Wormian bone (os antiepi-leptlcum) was present at the junction of the coronal and sagittal sutures (see fig. 1, p. 685). This bone has been described by Gruber and Howes, and I am inclined to regard it as of some little classificatory value l. It occurred twice in the five skulls examined, and I have never seen it in any other Kangaroo. (6) In Petrogale the palatine process of the palate-bone is only represented by a narrow bridge marking the posterior boundary of the hard palate. In Macropus the palatine process is complete and the back of the hard palate has no perforation of any size. (7) The premaxilla of Petrogale is a larger bone in comparison with that of Macropus, and has not the sharp angle running back between the maxilla and nasal found in the latter. (8) The inter-parietal bone of Petrogale has a very different appearance to that of Macropus ; in the latter it is a more or less crescentic bone having a much greater breadth from side to side than from before backward, while in the former its anteroposterior measurement equals its greatest transverse, so that the bone forms either an isosceles triangle or a rough pentagon. (9) The paroccipital processes are better developed in Macropus than in Petrogale. As special stress is laid on the condition of the centres ot ossification of the various bones, it will assist in determining the animal's age if the state of the teeth is noticed here. The anterior 1 See tbe author's paper on Atherura africana, P. Z. S. 1894, p. 6//. |
