VERTEBRATE LIFE IN AMERICA. 687 



Dryptodon has been found only in the Coryphodon beds of New Mex- 

 ico, while Tillotherium and Stylinodon occur in the middle Eocene of 

 Wyoming. Anchippodus probably belongs to this group, which may 

 perhaps include some other forms that have been named from frag- 

 mentary specimens. 



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

 quently 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 Sciuravus, which continued through- 

 out the Eocene. A nearly allied form, which may prove to be the 

 same, is Pararnys, the species of which are larger than those of the 

 older type. In the Dinoceras beds, the genus Colonomys is found, 

 and the specimens preserved point to the Muridce as the nearest living- 

 allies. A peculiar genus, Apatemys, which also occurs in the middle 

 Eocene, has gliriform incisors ; but the molars resemble those of In- 

 sectivores. All the Eocene Rodents known 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 

 Oreoclon beds, and continue in considerable numbers through the 

 rest of the Tertiary and Post-Tertiary, to the present day. In these 

 beds, the most common forms belong to the Leporidce, and mainly 

 to the genus Palceola'gus. The Squirrel family is represented by 

 Ischyromys, the Muridce by the genus Eumys, and the beavers by 

 Palmocastor. 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 

 Mountains, Rodents continue abundant; but most of them belong to 

 existing genera. Among these are Castor, Hystrix, Cynomys, Geomys, 

 Pepus, and Hesperomys. In the Post-Tertiary, the gigantic beaver, 

 Castor oides, was abundant throughout most of North America. Hy- 

 drochoerus has been found in South Carolina. In the caves of the 

 island of Anguilla, in the West Indies, remains of large extinct Ro- 

 dents, belonging to the ChinchilUdo?, have been discovered. 



The early Tertiary Rodents known from South America are the 

 genera Megamys, Theridromys, and a large species referred to Arvi- 

 cola. In Brazil, the Pliocene Rodents found are referred to the exist- 

 ing genera, Cavia, Kerodon, Lagostomus, Ctenomys, Hesperomys, Ox- 

 ymycterus, Arvicola, and Lepus. A new genus, Cardiodus, described 

 from this horizon, is a true rodent ; but the peculiar Typotherium, 

 which has been referred to this order by some authorities, has per- 

 haps other affinities. In the Post-Tertiary, the Rodents were very 

 abundant in South America, as they are at present. The species are, 

 in most instances, distinct from those now living, but the genera are 



