362 EXTINCT MAMMALIA OF DAKOTA AND NEBRASKA. 



The pliocene formation of the Niobrara is remarkable for the abundance of its 

 equine remains, which have been referred to five genera, of which Merycld'pjms and 

 Parahippus are peculiar, and Protohippus has been discovered elsewhere only in 

 South America. The remaining genera, Hipparion and Equus, belong also to the 

 North American quaternary and likewise to the European tertiary and quaternary 

 formations. 



The miocene rodents of the Mauvaises Terres belong to four peculiar genera, of as 

 many still existing families. One of the genera, however, Palceocastor, may be identi- 

 cal with the European Steneofiher, or Ghalicomys, of cotemporaneous age. 



The pliocene rodents of the Niobrara appear to belong to the still existing genera, 

 Castor and Hystrix, but the latter now exists only in the old world. 



Of the few discovered quaternary rodents of North America, one genus, Hydro- 

 chcerus, now absent on this continent, still lives in South America. 



The miocene Insectivora of North America belong to three genera, not discovered 

 elsewhere. 



