MAMMALS. 



379 



prolongation of the larynx into the choana, much as in the whales. 

 In development the ova pass into the uterus, from which they 

 absorb nourishment without the intervention of a 

 placenta (except in Perameles), no villi 

 being developed on the serosa, and 

 the allantois failing to reach this 

 envelope. An osteological pe- 

 culiarity, present in all except 

 in Tarsipes, is the inflection 

 of the posterior angle of the 

 jaw. 



In size the marsupials 

 vary from animals the size 

 of a rat, up to the giant 

 kangaroo ; while in the past 

 Diprotodon was as large as 



FIG. 357. Skeleton of kangaroo, from Macallister. 



a rhinoceros. In form and habits they show many modifica- 

 tions, usually attributed to the fact that in Australia they 

 have been removed from competition with other mammals, and 

 have developed in every direction, terrestrial, crawling, leap- 

 ing, climbing, and soaring forms. The majority are nocturnal. 



SUB-ORDER i. POLYPROTODONTIA. 



Incisors ? -, small, subequal ; canines larger; molars acutely tuber- 



4 or 3 

 culate. 



The DIDELPHID^E, opossums, American ; teeth i J, c \, p \ , /// ; feet all 

 five-toed ; tail partially naked and usually prehensile. Didelphys virginiana, 

 north to New England ; other species in the tropics. Chironectes has webbed 

 feet. Didelphys occurs in the eocene of France and America. DASYURID^E 



i -,c-,p~ 3 ;// ; hind feet four-toed. Thylacinus is dog-like, 



3 i ^ 2 or 3 4-6 



carnivorous, and occurs in Tasmania. Myrmecobius with ;// f , feeds on 

 ants. Dasyurus, Phascogale. Allied forms fossil in lower tertiary of South 

 America and later tertiary of Australia. The PERAMELID.E, i f , c {, p f ,. 



