708 



MAMMALIA. 



had three pairs of remarkable protuberances on the top of the skull, 

 no upper incisors, large upper canines, especially in the males, and 

 six back teeth. 



Example. — Uintatherium. 



Some Tertiary American forms, e.g. Toxodon and Nesodon, varying 

 in size from that of a sheep to that of a rhinoceros, form the sub-order 

 Toxodontia. 



Cope includes a number of generalised Eocene Ungulates under 

 the title Condylarthra. Some seem ancestral to the Perissodactyla 

 and Artiodactyla ; some suggest a union of ancestral Ungulates and 

 ancestral Carnivores. The genus Periptychus may be regarded as 

 an ancestral Bunodont, and Phenacodus as near the origin of the 

 horse stock. But Phenacodus is so generalised that Cope suggested 

 affinities between it and not only Ungulates, but also Carnivores and 

 Lemurs. 



From the Eocene of N. America, Marsh has disentombed a group 



Fig. 312. — External appearance of common porpoise. 



of animals which he calls Tillodontia, e.g. Tillotheritim, which 

 seem to combine the characters of the Ungulata, Rodentia, and 

 Carnivora. 



Few orders of Mammals are of more interest to the palaeon- 

 tologist than the Ungulates. Not only are fossil representatives 

 numerous, but their usually large size, and the fact that the teeth 

 are frequently an index of general structure, makes the determina- 

 tion of affinities much easier than in most cases. In consequence, 

 problems like that of the origin of the horse, or the relations of the 

 different proboscidians, have been worked out with a completeness 

 rare elsewhere. 



Order Cetacea. 



The Cetaceans, including whales and dolphins and 

 their numerous relatives, are aquatic mammals of fish-like 

 form. 



The spindle-shaped body has no distinct neck between 



