GEOLOGICAL SURVEY OF THE TEREITOKIES. 363 



BODENTIA. 



The order of gnawing animals is represented in the tertiary formations 

 of the West by half a dozen genera, most of which are extinct. The 

 comparative smallness of the animals of this order renders their remains 

 more liable to total destruction than those of larger animals, and also to 

 their escaping notice when preserved. No doubt many remains of un- 

 known species and genera will be discovered in future explorations of 

 the tertiary deposits. 



Leporidce. 



The hare family is represented in the miocene formation of the man- 

 vaises terres by a peculiar genus, which probably was the remote ances- 

 tor of our rabbits. 



PAL.3SOLAGUS. 



The genus is established on a number of fragments of jaws and teeth 

 discovered by Professor Hayden at the head of Bear Creek, a tributary 

 of the Sheyenne Eiver, Dakota. Palseolagus had the same number of 

 teeth as in the rabbit, but the first lower molar is composed of a double 

 column, as in the other molars, whereas in the latter it has an additional 

 column. 



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

 was rather less in size than our common gray rabbit. 



Sciuridce. 



The family of the squirrels and marmots was represented by a pecu- 

 liar genus, whose remains were discovered by Professor Hayden in 

 association with those of the former. Similar remains were subsequently 

 obtained from the mauvaises terres of White Eiver. ., The genus is 

 -distinguished by the following name : 



ISCHYROMY3. 



The skull of the genus approaches most nearly in form that of the 

 living beaver, but the teeth more nearly resemble those of the squirrel 



Ischyromys typus. — The species was about the size of the common 

 muskrat. 



Castoridce. 



The beaver family is represented both in the miocene and pliocene 

 formations of Dakota and Nebraska. 



PAL^OCASTOR. 



A rodent or gnawer, to which this name is given, is founded on several 

 incomplete skulls, together with a number of fragments of jaws with 

 teeth, obtained by Professor Hayden in the miocene deposit of the 

 mauvaises terres of White Eiver, Dakota. 



Palceocastor nebrascensis. — The species was about half the size of the 

 existing beaver. 



CASTOR. 



The sole representatives of this genus now living are the American 

 and the European beaver. 



Castor tortus. — An extinct species, thus named, is indicated by a por- 

 tion of a skull, with teeth, found by Professor Hayden iu the pliocene 

 sands of the Mobrara Eiver. It was only half the size of the living 

 spocies. 



