THE EOCENE OF EUROPE AND NORTH AMERICA 161 



tives of the characteristically South American order of side-necked or 

 pleurodiran turtles, but such have not been found. The soft-shelled river 

 turtles (Trionychoidea) Avcre represented by at least two species, whereas 

 there are at present in the whole world only twenty-six; the Bridger rivers 

 and brooks fairly swarmed with these creatures, some of them equal in 

 size to the largest existing Asiatic species. They are indicative of flowing 

 waters. Swampy conditions are indicated by the presence of fourteen 

 species of the family EmydicUe (order Cryptodira), as compared with the 

 twelve species living in the Mississippi valley to-day. The genus Baptemys 

 of the same onUu- has its nearest relatives at present in Central America, 

 while a third genus {Anosteira) is reported by Lydekker in the Upper 

 Eocene of England. The presence of extensive stretches of land is indi- 

 cated by the true land tortoises (Testudinidse) of the genus Hadrianus, 

 including giant tortoises nearly three feet in length, which probably lived 

 on dry lands bordering the Bridger Basin. The ancient, Lower Cretaceous 

 order Amphichelydia is also represented here by four species belonging to 

 two genera. 



The Succession of Middle Eocene Mammals 



Lower Bridger or Orohippus Zone. — The Lower Bridger includes levels 

 A, B, C, of the section Fig. 58. It is characterized by the absence of the 

 Amblypoda. It may be distinguished as the Orohippus Zone from the 

 presence of these characteristic little 'mountain horses.' 



Many older Wasatch and Wind River species are found on this level 

 which do not survive into the Upper Bridger. All the creodonts and pro- 

 Carnivora are of older type. The Insectivora, Primates, and smaller Car- 

 nivora are richly represented in the locality kno^\^l as Grizzly Buttes. 

 Here have been found among the monkeys, or lemurs, the Tarsius-like 

 anaptomorphids, the notharctids, resembling some of the South Ameri- 

 can monkeys, as well as remains of Microsyops with its rodent-like incisors, 

 analogous to those of the aye-aye of Madagascar. There is also the long- 

 tailed and probably aquatic insectivore Pantolestes, and numerous minute, 

 shrew-like insectivores. Opossums (Peratherium) also occur. The primi- 

 tive hyaenodonts Sinopa and Tritemnodon abound in these lower beds. 

 Mesonyx is a Lower Bridger animal. 



Among the hoofed mammals or ungulates the delicate lophiodont 

 Helaletes is most common, analogous to the Chasmotherium of France. 

 Among the horses five species of Orohippus occur, all animals of small 

 size and still possessing four digits on the fore feet. Among titanotheres 

 principally the broad-headed Falceosyops and Limnohyops occur in rela- 

 tively early stages of evolution. It is noteworthy that no traces of horns 

 are found upon the heads of the titanotheres on this level. The pro- 

 rhinoceroses, or hyracodonts, are represented only by the light-limbed 



