xm 



PHYLUM CHORBATA 



587 



maximum in the existing genus Equus. The history of this reduc- 

 tion, together with the development of other characteristic features, 

 can be traced from pentadactyle forms with simple molars through 

 a long series of gradations to the monodactyle Horses with their 

 complexly folded molars. Similar genealogies, though not always 

 so complete, can be traced for the Tapirs and Rhinoceroses, and for 

 the Deer, Camels, and Pigs. 



The order Proboscidea was represented in Tertiary and 

 Pleistocene times, not only by forms allied to those now living 

 though sometimes, as in the Mammoths, of much greater 

 size but by an extinct family, the Dinotheridce (Fig. 1247) (Mio- 

 cene and Plio- 

 cene of Europe 

 and India), 

 which possess a 

 pair of down- 

 wardly - directed 

 tusks in the 

 lower jaw. The 

 genus Pyrothe- 

 rium, from the 

 Patagonian Ter- 

 tiary deposits, 

 was a primitive 

 Ungulate, with a 

 pair of short 

 tusk-like incisors 

 in the lower jaw, 

 which may have 

 been related to 

 the Proboscidea : 

 and Mcerithe- 

 rium, from the 

 Eocene of Egypt, 

 is probably also 

 a primitive 



JX ~t 4-1, ~ Fio. 1247. Dinotherium giganteum. Side view of skull. 

 T 6 th natural size. (From Zittel's Palaeontology, after Kaup.) 



same order. 



The Hyracoidea were represented in the Pliocene by an extinct 

 genus (Pliohyrax) ; and Archceohyrax, from the Patagonian Tertiary, 

 is perhaps also an allied form. 



A separate sub-order, the Condylarthra, has been established 

 for a number of Eocene Ungulates, which differ somewhat widely 

 from all the other members of that group, and approach the 

 Carnivora in some respects, though having certain resemblances to 

 the Hyracoidea. The pre-molars and molars are short and usually 

 bunodont, the pre-molars being simpler than the molars, the latter 



