| OCR Text |
Show 1900. | FOSSIL MARSUPIAL FROM TASMANIA. 783 plates which are attached to but not fused with the skull-wall. It thus differs markedly from that of other marsupials, in almost all of which there is a well-marked pit for the lodgment of the lateral appendage of the cerebellum. In the W o m b a t the pit is represented by a depression, but there is no horizontal platform developed, and the whole structure is very different from that of the fossil. The cranial cavity is relatively of considerable size as compared with that of recent marsupials. Some idea of this m a y be gained from the following measurements ', which, for the sake of more easy comparison, have all been reduced so as to make them proportionate in dimension to skulls of the same length 2 as that of the fossil specimen (100 m m . ) : - Total length of 1 cranial cavity ... J Greatest height ... Length of cerebral 1 Length of cerebellar 1 Greatest width of 1 cerebral fossa ... | "33 OQ o 58 26 no OO 125 33 Trichosurus vitlpecula. 505 21 30-5 13-7 294 s « 504 20-4 30-7 10-8 28-9 Macropus rujus. 44-8 24 32 10-3 29 421 205 275 9 28 Perameles gunni. 44 21-3 26-3 9-3 28 CO H J* 426 18-2 23-4 8-6 23-4 It will be evident from these figures, which represent approximately the cranial development in typical examples of the families Macropodidae, Phalangeridae, Dasyuridae, Pbascolomyidae, and Peramelidae, that in the extinct form we have an animal in which the relative size of the brain was greater than in existing marsupials ; in the total length of the brain, and in the height, length, and breadth of the cerebral hemispheres, it has decidedly the advantage, and as indicating a possible retrogression in cranial development within the marsupial group since Eocene times, the fossil is of peculiar interest. 1 The cranial cavity has been cleared of matrix since the drawings were made. 2 This measurement is along the dorsal surface from the front end of the sagittal crest to the tip of the nasal bones, as, owing to the broken under surface, the length from basion to gnathion cannot be taken. In the case of the Kangaroo the difference between the latter and the dorsal length is greater than in the other forms, and causes the relative dimensions of the skull-cavity to be slightly greater than they would be if the more satisfactory measurement of gnathion to basion had been available. |
