DAKOTA AND NEBRASKA. 331 



RODENTIA. 



This order is represented by six species of as many genera, in the tertiary deposits 

 of the Mauvaises Terres of White River, and those of the Niobrara River. Four of 

 the genera are extinct, and belong to the miocene formation ; the others apparently 

 are referrable to Gastor and Eystrix, and belong to tlie pliocene formation. They 

 represent five families, all still in existence : the Leporidiv, Sciuridce, Castoridce, 

 Muridce, and the Hystricidce. 



LEPORIDM. 



The Hare family is represented in the miocene fauna of the Mauvaises Terres by a 

 peculiar genus, which has been named as follows : 



PAL^OLAGUS. 

 Pal^olagus Hatdeni. 



The genus and species are founded upon a number of fragments of upper and lower 

 jaws with teeth. The specimens were discovered by Dr. Hayden at the head of Bear 

 Creek, a tributary of the Sheyenne River, Dakota. They are considered by Dr. 

 Hayden to be of miocene age, and belong to bed C of his vertical section of the 

 tertiary deposits of Dakota and Nebraska. 



The specimens indicate Palceolagm to have had the same number of molar teeth 

 as the existing Hares or Rabbits, — six to the upper and five to the lower series. The 

 constitution of the teeth likewise is of the same character as in the latter animals. 

 In Palceolagns, however, the first inferior molar is composed of a double column as in 

 the others, whereas in the Hare it is a triple column. 



The bottoms of the inferior incisors, seen in several of the fossils, are noticed ex- 

 tending further back than in the Hare, reaching partly beneath and partly internal 

 to the anterior three molars, while in the Hare they stop short of the position of the 

 first molar. 



From the recent genus Lagomys, and the extinct genus Titanomys of the miocene 

 deposits of France and Germany, Palceolagus differs, as does the Hare, in the pos- 

 session of a greater number of molar teeth. 



The fossils indicate a species rather less in size than the Gray Rabbit, Lepus syl- 



