43 



may perhaps include some other forms that have been named 

 from fragmentary specimens. 



The Rodents are an ancient type, and their remains are not 

 unfrequeutly disinterred in the strata of our lowest fresh-water 

 Eocene. The earliest known forms are apparently all related 

 to the Squirrels, and the most common genus is Sciwravus, 

 which continued throughout the Eocene. A nearly allied form, 

 which may prove to be the same, is Paramys, the species of 

 which are larger than those of the older type. In the Dino- 

 ceras beds, the genus Colonomys is found, and the specimens 

 preserved point to the Muridce, as the nearest living allies. A 

 peculiar genus, Apaiemys, which also occurs in the middle 

 Eocene, has gliriform incisors, but the molars resemble those of 

 Insectivores. All the Eocene Rodents are of small size, the 

 largest being about as large as a rabbit. 



In the middle and upper Miocene lake-basins of the West, 

 Rodents abound, but all are of moderate size. The Hares first 

 appear in the Oreodon beds, and continue in considerable num- 

 bers through the rest of the Tertiary and Post-Tertiary to the 

 present day. In these beds, the most common forms belong to 

 the Leporidw, and mainly to the genus Palocolagus. The Squirrel 

 family is represented by Ischyromys, the Muridce by the genus 

 Eumys, and the Beavers by Palceocastor. In the upper Miocene 

 of Oregon, most of the same genera are found, and with them 

 some peculiar forms, very unlike anything now living. One of 

 these is the genus Allomys* possibly related to the flying 

 Squirrels, but having molar teeth somewhat like those of the 

 Ungulates. In the Pliocene, east and west of the Rocky Moun- 

 tains, Rodents continue abundant, but most of them belong to 

 existing genera. Among these are Castor, Hystrioc, Cynomys, 

 Geomys, Lepus and Hesperomys, In the Post-Tertiary, the 

 gigantic beaver, Castoroides, was abundant throughout most of 

 North America. Hydrochcerus has been found in South Caro- 

 lina. In the caves of the island of Anguilla, in the West 

 Indies, remains of large extinct Rodents belonging to the Chin- 

 cliillidce have been discovered. 



