SUCCESSIVE MAMMALIAN FAUNAS 275 



The Wind River genus {\Bathyopsis) was a very much smaller 

 animal than any of the Bridger forms and its horn-like pro- 

 tuberances were in an incipient state, while in various other 

 respects it was decidedly more primitive than its successors. 

 The second family was represented by the genus '\Corijphodon, 

 which did not survive into the Bridger, but was especially 

 characteristic of the Wasatch fauna, with which it will be 

 described. 



Turning now to the progressive orders, we note that the 

 rodents, lemurs and monkej^s were very similar to those of 

 the Bridger and belonged to the same families, but were 

 decidedly less numerous. This difference, however, may be 

 rather apparent than real and due to the much more favourable 

 conditions for the preservation of small mammals in the middle 

 Eocene. Among the Perissodactyla, the horses were inter- 

 mediate in size and structure between those of the Bridger 

 and those of the Wasatch, but were decidedly nearer to the latter. 

 The flophiodonts, so far as known, were represented by a single 

 genus {]Heptodon) which also occurred in the Wasatch. The 

 modest beginnings of the ftitanotheres, the family which be- 

 came so very conspicuous in the middle and upper Eocene 

 and lowest Oligocene, may be noted in the Wind River fauna, 

 in which there were two genera. One of these {'\Eotitanops), 

 the very probable ancestor of all the subsequent genera, was 

 quite small, about two-thirds the size of a modern tapir, while 

 the other {^Lamhdotherium) was a much smaller, lighter and 

 more slender animal and apparently belonged to an abortive, 

 short-hved phylum. Then, too, the first of the fhyracodonts, 

 or cursorial rhinoceroses, made their appearance here in the 

 genus ^Hyrachyus, which was afterward so common in the 

 Bridger. 



No Artiodactyla have yet been found in the Wind River, 

 though there can be little doubt that they then inhabited North 

 America, as they did both before and afterward. 



The Wind River fauna was of so much less peculiar and 



