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Show 794 ON A FOSSIL MARSUPIAL FROM TASMANIA. [NOV. 20, hand with existing Polvprotodontia, and on the other with existing Diprotodontia. If w e had only the anterior part of the skull preserved, there is but little doubt that it would be referred to tbe Phalangeridae; but, on the other hand, if w e had only the hinder part showing the strong sagittal crest, the low origin and wide sweep of the zygomatic arch, it would be referred to the Dasyuridae; the ilium alone would be regarded as belonging to an animal more allied to Dendrolagus than to any existing marsupial; while the head of the tibula would be regarded as indicating affinity to Phascolomys. W e may divide the more important characters into three groups as follows :- (1) Those in which it differs from recent, marsupials. These characters are seen in numbers 9, 13, 15, 17, 22, 32, 35, 36. The most important features in connection with these is the greater relative size of the cranial cavity, in conjunction with tbe fact that the greatest breadth of the frontal bones lies in the cranial and not in the facial region. There is no indication of affinity with any special group outside of the marsupials, but an indication that within the marsupials retrogression has most probably taken place in this respect. (2) Those in which it shows alliance with the Polyprotodontia. These are seen in numbers 1, 2, 3, 5, 7, in all of which it shows a decided alliance with the Dasyuridae. (3) Those in which it shows alliance with the Diprotodontia. These are seen in numbers 8 (to a certain extent), 12 (to a certain extent, though this character is common to DidelphyidaB), 16,19,28, 29,30, 31, 32. In some of these features it shows alliance with the Phalangeridae and in others with Phascolomys, and in 32 with Macropodidae to a certain extent. In the complete absence of teeth and of the bones of both fore and hind feet, it is unsafe to attempt to refer it to any existing family; there are, indeed, only two which could possibly be considered in this respect-the Dasyuridae and Phalangeridae. From both of these it differs in the points mentioned in the first group : from the Dasyuridae in the characters mentioned in the third group as well as in 4 (in part), 18, 32, and 33; from the Phalangeridae in those of the second group as well as in 4, 6, 18, 28, 32, 33, 34. The structure of the premaxilla? appears to indicate the existence of a fair-sized incisor dentition; and a consideration of all the features would appear to lead to the conclusion that the fossil is the representative of a now extinct series of forms which were more nearly allied to ancestral Polyprotodonts than are any of the existing Diprotodont forms. It may, in fact, be regarded as intermediate between the former and the latter, and as indicative of a stage in the development of Australian marsupials when the ancestors of the recent Diprotodontia were beginning to diverge from the original Polyprotodontid stock from which they have been developed within the limits of the Australian region. Melbourne, May 1900. |