BY R. BROOM. 319 



which is protected by a well developed, thin alisphenoid 

 bulla. An exactly similar condition is found in the Didel- 

 phids, and a somewhat similar condition in tliu Dasyurids 

 and Peramelids. The arrangement of the foramina in the 

 squamosal bone, and the relations of the sqiiamosal to the 

 periotic are all typical Polyprotodont characters. 



Ill the relationships of the bones, and the foramina in the 

 posterior basicranial region, Ccenolestes closely resembles the 

 primitive Didelphids, such as Marmosa, and less closely the 

 Australian Polyprotodonts ; while the differences from the 

 conditions in the Diprotodonts are considerable. 



Apart from the condition of the teeth, Ccenolestes is a 

 typical Polyprotodont in all its cranial characters, and the 

 question to be considered is whether the Diprotodont-like 

 character of the teeth is of sufficient weight to place Cceno- 

 lestes among the Diprotodonts, in spite of the cranial charac- 

 ters all pointing in the otlier direction. 



Now while a type of dentition may remain practically 

 unaltered throughout long ages, if the habit remains the 

 same, it is surprising how readily the type may be altered 

 with change of habit. Thus in the Didelphids we find a 

 dentition which has remained with little change throughout 

 the Tertiarv i)eriod. Biit the Diprotodonts of Australia, 

 though closely related, have, in probably a very much shorter 

 time, evolved in a number of very different ways. Further, 

 numerous instances can be given of animals in no way nearly 

 related to each other, evolving closely similar types of den- 

 tition. For example, take the molars of Notoryctes and 

 Chrysochloris, or of Diprotodon and Dinotheriuw . And a 

 Diprotodont arrangement has been independently evolved in 

 in a large number of the mammalian orders, e.;/., Multituber- 

 culata, Rodentia, Ungulata, Primates, Chiroptera, and even 

 to some extent in the Insectivora. 



Though in Cienolestes the dentition bears a superficial 

 resemblance to that of the Diprotodonts, it really differs in 

 some important points. In the first place, no known Diproto- 



