432 University of California Publications in Geology [Vol. 10 



The fauna reported from beds referred to the Idaho formation is 

 as follows : 



Fauna Referred to Idaho 

 Equus idahoensis, n. sp. 

 Equus excelsus? Leidy 

 Protohippus ? 



Rhinoceros, probably Aphelops (Teleoeefras) fossiger (Cope) 

 Mastodon miriflcus Leidy 

 Procamelus, size of P. major (Leidy) 



Cervus, possibly new, slightly smaller and more slender than C. canadensis 



(Erxleben) 

 Horn-core of true antelope (Tragocerus?) 

 Ischyrosmilus, n. sp. 

 Morotherium leptonyx Marsh 

 Castor, possibly n. sp. 

 — Olor, size of O. palaeocygnus 

 Graeulus idahoensis Marsh 



The Idaho formation is not as yet satisfactorily separated from 

 the Payette Eocene or Miocene, and from a Miocene or Pliocene stage 

 which may intervene between the Payette and the Idaho. It is, how- 

 ever, quite certain that there exists over a large area of southwestern 

 Idaho a formation several hundred feet thick which may show evidence 

 of deformation, and which contains a fauna of a stage representing 

 either the latest Pliocene or the earliest Pleistocene. 



Mammalian remains presumed to represent the Idaho have been 

 reported from a number of localities of which the relative geologic 

 position is uncertain. One might assume from the composition of the 

 entire list of forms obtained by Lindgren that the collections are in 

 the main from beds of Pliocene age, with the possibility that some of 

 the elements are derived from Pleistocene deposits, and possibly some 

 from horizons older than typical Idaho. 



Of the several mammalian types listed by Lindgren, the horses are 

 undoubtedly the most important for palaeontologic determination of 

 age. The only form specifically determined in the earlier collections 

 was one considered by Leidy to represent Equus excelsus. This species 

 is not known from beds older than Pleistocene. Bones from near 

 Sommercamp Ranch, in the northern portion of the Silver City quad- 

 rangle, have been referred to Protohippus. If this determination is 

 correct, these beds might be Pliocene or late Miocene. Horse remains 

 obtained at Idaho localities by Buwalda and Merriam in 1916, in the 

 course of investigations carried on for the United States Geological 

 Survey, are of a type clearly to be referred to Equus. In the lowest 

 beds examined at a locality on the Snake River, southwest of the town 



