UNGULATA. 299 



have attained a length of about three metres; while other species are 

 recognized in the same formation. Tinoceras (fig. 170), also from the 

 Bridger Eocene of Wyoming, differs very little from Ditwceras; while 

 i'intatherium, based on specimens too imperfect for precise determination, 

 from the same formation and locality, is not improbably identical with the 

 latter. 



Sub -Order 4. Proboscidea. 



The existing elephants represent, a sub-order of hoofed 

 quadrupeds which have retained nearly all the essentially 

 primitive characters of the order, except those directly con- 

 nected with the well-known unique mode of feeding which 

 they have acquired. Apart from the possession of (i.) a trunk 

 or proboscis, (ii.) tusks instead of incisors, and (iii.) a grinding 

 dentition of abnormal type, they are indeed the least special- 

 ized among surviving Ungulata. Although the brain is of 

 considerable size and the cerebral hemispheres exhibit numerous 

 convolutions, these hemispheres do not cover the cerebellum, 

 which is entirely posterior to them. In the circulatory system, 

 there are two anterior venae cavse still persisting. In the 

 skeleton, the limb-bones are adapted solely for the support of 

 the massive body, with a separate and complete radius and 

 ulna, a separate and complete tibia and fibula, and five hoofed 

 digits on each foot. In the carpus, the bones of the two rows 

 are still directly opposed to each other, and not alternating ; 

 while in the tarsus, the astragalus is flattened, not tongued or 

 grooved on its articular face for the tibia. 



So far as can be inferred from the disposition of the nasal 

 bones, the cranial roof, and the shortened neck, the Proboscidea 

 were already differentiated from the other hoofed quadrupeds 

 by the possession of a prehensile trunk so long ago as the early 

 part of the Miocene period. No more ancient forms have 

 hitherto been identified, and no known group of Eocene 

 Ungulata can yet be claimed with much plausibility as directly 

 ancestral to them. Pyrotherium, from deposits which seem 

 to date back to an early Tertiary period in Patagonia, is 

 sometimes claimed to be a primitive Proboscidean ; but 

 its affinities still remain to be demonstrated. From the 

 Middle Miocene onwards, however, many fine examples of the 



