364 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Muridce. 



The rat family is represented in the miocene deposit of the mau- 

 vaises terres of White Eiver by an extinct genus, to which the following 

 name has been applied : 



ETTMYS. 



This genus is founded upon the fragment of a jaw discovered by Pro- 

 fessor Hayden. 



Eumys elegans. — The species was about the size of the brown rat. 



Hystricidce. 



The porcupine family is represented by a species whose remains were 

 found by Professor Hayden in the pliocene sands of the Mobrara Eiver. 



HYSTEIX. 



The living species of this genus are confined to the Old World, none 

 having been discovered in America. 



Hystrix venustus. — An extinct species, indicated by several isolated 

 teeth from the Niobrara Eiver. Judging from the characters of the teeth 

 it was more nearly related with the crested porcupine of Europe than 

 with our own living porcupine, pertaining to another genus. 



INSECTIYOEA. 



The insectivorous order of animals, mainly consisting of the smallest 

 forms of the mammalian class, as might be supposed, are among the 

 rarest of those whose remains are found preserved in fossiliferous 

 strata. Professor Hay den's explorations in the West have led to the dis- 

 covery of three extinct genera of the order, pertaining to the miocene 

 tertiary formation of the mauvaises terres of White Eiver, Dakota, 

 and a third genus was discovered by Dr. J. Yan A. Carter, of Port 

 Bridger, Wyoming, in the Bridger Group of tertiary deposits of his 

 vicinity. 



LEPTICTIS. 



The genus to which this name is given is established on the speci- 

 men of an almost complete skull, which was found imbedded in a soft 

 rock near the mouth of one of the small tributaries of White Eiver, in 

 the mauvaises terres. It possessed three incisor, a canine, and seven 

 molar teeth on each side of the jaws. The top of the skull presents a 

 pair of prominent ridges, defining the temporal fossse. The animal was 

 allied more nearly to the hedgehogs than to any other of the living 

 members of the order. 



Leptictis Haydeni. — The species, named in honor of its discoverer, was 

 scarcely as large as a mink. The skull is barely two and a half inches 

 long. 



ICTOPS. 



A second genus, closely allied to the preceding, is indicated by a small 

 fragment of a skull, found in association with the former specimen. It is 

 distinguishable from it by difference in the form of the molar teeth. 



Ictops dalcotensis. — A species about the same size as the preceding, 

 with which it lived cotemporaneously. 



OMOMYS. 



This extinct genus, probably pertaining to the same family as the 

 European hedgehog, was established on .the fragment of a lower jaw, 

 with teeth, from the tertiary formation near Fort Bridger, Wyoming. 



