Vol. 6] Merriam: Virgin Valley and Thousand Creek. 215 



From the lists available, it is evident that the faunas of Virgin 

 Valley and Thousand Creek represent distinct epochs in the 

 evolution of the mammalia of western North America. Between 

 the times of the deposition of the two series of deposits sweep- 

 ing changes in the fauna of this region had taken place. The 

 only species known to persist from Virgin Valley to Thousand 

 Creek time are three rodents; two of which, Aplodontia and 

 Lepus, represent extraordinarily persistent genera. The third 

 form, Mylagaulus, has a range from Middle Miocene to Pliocene. 

 The single M 2 of Tephrocyon found at locality 1103 does not 

 differ markedly from the corresponding tooth of T. kelloggi from 

 Virgin Valley. Other than these species there are no forms 

 which appear to be common to the two series of beds. The masto- 

 don, a large cat, and perhaps some of the camels, may be similar 

 in the two formations, but the material available is not sufficient 

 for specific comparison. 



The possible elements common to the Thousand Creek and 

 Virgin Valley Beds are the following, of which the last three are 

 very doubtful and the fourth uncertain. 



Aplodontia alexandrae Furlong. 

 Mylagaulus monodon Cope. 

 Lepus vetus Kellogg. 

 Tephrocyon, near kelloggi, n. sp. 

 Felis, sp. a ( f ) 



Mastodon (Tetrabelodon ?, sp.) 

 Camelid(?) 



The ungulates may presumably be fairly taken as a basis for 

 comparison of the two faunas, inasmuch as they include a large 

 percentage of the species known, and are, moreover, the most 

 abundantly represented among the specimens collected in the 

 two regions. In this group we find Hypohippus, ParaMppus, 

 Merychippus, Moropus, Merychyus, Dromomeryx, Blastomeryx, 

 and Merycodus of the Virgin Valley fauna entirely unrepre- 

 sented in the Thousand Creek Beds. At Thousand Creek horses 

 of the Pliohippus type are the common forms; the only other 

 remains of this group known are the few tentatively referred to 

 Equus. Among the artiodactyls a small dieotyline from Thou- 



