EOCENE 505 



a close general similarity in their mammals. The southern conti- 

 nents, on the other hand, had altogether different mammalian 

 faunas, due to their long separation from the northern lands. 



The Puerco shows its close relations with the Mesozoic in the 

 presence of numerous Multituberculata, the last and largest of 

 that group. (Ptilodus and Polymastodon are the common genera.) 

 The primitive type of flesh-eaters (Creodonta) and ancestors of 

 the true Carnivores are abundant, as are also the primitive hoofed 

 animals (Condylart/ira and Amblypoda) ; the curious Tillodonts, 

 Ganodonts, and primaeval Lemuroids complete the list. Espe- 

 cially noteworthy is the entire absence of Rodents, of true Carni- 

 vores, of Artiodactyls, and Perissodactyls. 



The change to the Wasatch is very abrupt, and was probably 

 due to a great migration of mammals from some region, as yet 

 unknown. Rodents come in for the first time in North America. 

 Perissodactyls make their first appearance with ancestral members 

 of the horse family {Hyracotherium) , of the rhinoceroses (Hepto- 

 doti), the tapirs (Syste?nodon) , and other families now extinct. 

 The curious extinct group of hoofed animals called the Ambly- 

 poda greatly increases in numbers and in stature, and both in 

 Europe and America the predominant genus is Coryphodon. 

 Artiodactyls also appear for the first time in ancestral members 

 of the pigs (Eohyics), and the ruminants {Trigonolestes) . The 

 Creodonts increase in numbers and in the size and strength of the 

 individuals, Pachycena being as large as a bear, and Oxycena was 

 an aquatic form. Numerous Lemuroids and primitive types of 

 Monkeys {Anaptomorphus) swarmed in the trees. The corre- 

 spondence between the mammals of Europe and North America 

 was never closer than in Wasatch times. 



Between the Wasatch and Bridger lie the Green River Shales, 



Explanation of Plate XI, p. 504. 1. Ostrea virginiana, 1/2, Miocene. 

 (Whitfield.) 2. Pecten madisonicus, 1/2, Miocene. (Whitfield.) 3. Cardita per- 

 antiqua, Eocene. (Whitfield.) 4. Volutolithes sayana, 3/4, Eocene. (Whitfield.) 

 5. Oliva carolinensis, 3/4, Miocene. (Whitfield.) 6. Helix Dalli, Miocene. 

 (White.) 7. Planorbis convoluta, ? Fort Union. (Meek.) 8. Aturia Vanuxemi, 

 1/4, Eocene. (Whitfield.) 9. Glyptostrobus Ungeri, 1/2, Eocene. (Lesquereux.) 

 10. Salix sp. ( 3/4, Oligocene. 



