86o PHYLUM CHORDATA : CLASS MAMMALIA 



America. The molar teeth show transitions between those of elephants 

 and those of other Ungulates. 



In Dinotherium, found in Miocene and Pliocene strata in Europe 

 and Asia, the lower jaw bore an enormous pair of tusks projecting 

 vertically downwards, and all the back teeth seem to have been in use 

 at the same time. 



Several Extinct Sub-Orders 



Although we cannot describe the following remarkable types, it is 

 important to notice their existence, for they serve to impress us with 

 the original connectedness of what are now separate orders. 



The huge Amblypoda, in Eocene formations in America and Europe, 

 had usually remarkable protuberances on the top of the skull, a very 

 small brain, large upper canines, especially in the males, and six back 

 teeth. 



Example. — Uintatherium (Dinoceras), with no upper incisors. 



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 Periptychns 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 disentombed a group of 

 animals which he called Tillodontia, e.g. Tillotherium, which seem 

 to combine the characters of the LTngulata, Rodentia. and Carnivora. 



Few orders of Mammals are of more interest to the palaeontologist 

 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 determination 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 pro- 

 boscidians, 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 torpedo-shaped body has no distinct neck between 

 the relatively large head and the trunk, and tapers to a 

 notched tail, horizontally flattened into flukes. The fore- 

 limbs are paddle-like flippers, and there are no external 



