350 THE AGE OF MAMMALS 



the main mass of the 'Republican River' Formation. Several museums 

 have secured materials of Teleoceras from this quarry sufficient to assemble 

 the scattered bones into complete skeletons. The skeleton mounted in the 

 American Museum of Natural History is shown in Fig. 161. The rhinoceros 

 bones lie on the bottom layer of the 'Quarry/ mingled with sand about two 

 feet in thickness; the heavy short bones of the feet and limbs lie at the 

 very bottom; the skulls, arch bones, and vertebrae lie higher up. For this 

 reason Sternberg is convinced that this was a quicksand deposit. 



Some miles to the east is another locality in which remains of rhinoceroses 

 and mastodons were found associated with those of large land tortoises (p. 

 342). As described by the last-named explorer, these tortoises were em- 

 bedded together in a space 150 feet in length and some four feet in thick- 

 ness; they were all found in normal position with plastron down, the heads 

 and limbs attached. There is thus considerable evidence that this was 

 part of a great assemblage of tortoises which had been overwhelmed by a 

 sandstorm and died where they were entombed.^ Another interpretation, 

 by Hay, is that these reptiles had burrowed into the sand to hibernate; 

 but this would hardly account for their facing in the same direction. 



Characteristic Lower Pliocene Mammals 



Multiple phyla of horses. — The Great Plains at this time were covered 

 with great herds of horses of many different kinds. The browsing section 

 is represented by Hypohippus, the last representative of the ancient anchi- 

 therine phylum of horses, with three toes and short-crowned teeth adapted 

 to browsing. It is distinguished by nearly perfect transverse crests on the 

 grinders, somewhat like those of early palaeotheres. 



The protohippine section as distinguished by Gidley - includes horses 

 with three toes and long-crowned teeth, adapted to grazing; it subdivides 

 into more primitive forms with subhypsodont teeth, such as Merychippus, 

 and more progressive forms with long-crowned, well-cemented teeth, such 

 as Protohippus and Pliohippus. The two latter animals are distinguished 

 by the diverse characters of the preorbital fossae on the sides of the face. 

 In Protohippus these two fossae are shallow, without sharply defined bor- 

 ders, while in Pliohippus there are two large and partly confluent fossae, or 

 depressions in front of the orbits, with sharply defined posterior borders. 

 It is generally believed that the true horse (Equus) has descended from some 

 more conservative or central forms, like Protohippus, but the species bridg- 

 ing the transition between Protohippus and Equus still await discovery. 



A fourth and distinct line of Pliocene horses is that which contains the 

 hipparions (Neohipparion), in which the antero-internal pillar of the pre- 

 molars (protocone) is completely separated from the transverse crests. 



' Sternberg, C. H., Letter. 



* Gidley, J. W., Revision of the Miocene and Pliocene Equidae of North America. Bull. 

 Amer. Mus. Nat. Hist., Vol. XXIIL Art. xxxv, Nov. 26, 1907, pp. 865-934.. 



