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 Wombat 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 may be gained 

 from the following measurements ', which, for the sake of more 

 easy comparison, have all been reduced so as to make them pro- 

 portionate in dimension to skulls of the same length ~ as that of 

 the fossil specimen (100 mm.) : — 



It will be evident from these figures, which represent approxi- 

 mately the cranial development in typical examples of the families 

 Macropodidse, Phalangeridse, Dasyuriche, Phascolomyida?, and 

 Peramelidae, that in the extinct form we have an animal in which 

 the relative size of the brain was greater than in existing mar- 

 supials ; 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. 



