1420 



CLASS MAMMALIA. 



cene. An allied genus is Chalicomys (Steneojiber), in which the 

 humerus has a foramen, and the molars (fig. 1293, c) are more dis- 

 tinctly rooted, with shallower enamel-folds ; it occurs in Europe in 

 the Middle and Lower Miocene, and also in the Miocene of North 

 America ; Palceocastor from the latter deposits being apparently 

 closely allied. 



Family Ischyromyid^e. — Ischyromys (with which Tillomys is 

 apparently identical) is a North American Miocene genus with 



Pm. — , M. -; having dental characters of the Sciuridce, but other- 



1 3 . 



wise resembling the Hystricidce. and Caviidce, and thus indicating 



that the sections into which the existing Rodents are divided will 

 not hold good for all the fossil forms. Pseudotomus from the White 

 River Miocene is an allied form ; while Sciuromys of the Quercy 

 Phosphorites is probably also related. Mysops of the North Ameri- 

 can Eocene, according to Dr Schlosser, may be the same as Ischy- 

 romys. Gymnoptychus (p. 142 1) may also be allied. 



Family Sciurid^. — In this family the cheek-teeth are usually 



2 "3 



Pm. — , M. - , but in Sciurus the first upper premolar is often absent. 



1 3 



The molars are rooted, and, with the exception of Eupetaurus of 

 Kashmir, are brachydont ; those of the upper jaw usually having 

 triangular or squared crowns, with two or more shallow infolds of 



Fig. 1298. — Side view of the skull of Cyno7nys Ludovicianus. 



enamel from the outer side (fig. 1299). The Marmots and their 

 allies (fig. 1298) have uncompressed incisors ; and the type genus 

 Arctomys is represented by the existing A. marmotta (fig. 1299) in 

 the Pleistocene of Europe, and by another species in that of 

 North America. Plesiarctomys (with which Sciuravus, Marsh, and 

 Paramys, Leidy, are identical) occurs in the Middle Tertiaries of 

 both Europe and the United States, and connects Arctomys with 

 Sciurus. Spermophilus, or the Sousliks, has both living and extinct 



