1416 



CLASS MAMMALIA. 



Fig. 1293. — A, Left lower cheek - teeth of 

 Archceomys; b, Left upper cheek-teeth of 

 Theridomys ; c, Left lower cheek-teeth of 

 Ckalicomys. A and b enlarged. In b the 

 uppermost, and in A and c the lowest, tooth is 

 the premolar. 



Theridomys (fig. 1293, b) they are rooted, and have three or 

 four re-entering enamel-folds, which form isolated disks on the 



worn crowns. This genus occurs 

 in the Lower Miocene and Upper 

 Eocene of Europe ; while Syl- 

 lophodus of the North American 

 Miocene is closely allied. Prot- 

 echinomys of the Lower Miocene 

 and Upper Eocene of France has 

 rooted cheek-teeth with crowns 

 very like those of Theridomys 

 and the living Echinomys (Oclo- 

 dontidce) ; while Archceomys of 

 the same deposits has the teeth 

 devoid of roots, with their en- 

 amel-folds continuing across the 

 crown, and dividing it into 

 laminae (fig. 1293, a). Treehomys 

 is a fourth allied genus from the 

 Quercy Phosphorites, having cheek-teeth somewhat like those of 

 Theridomys, but with laterally-compressed crowns. 



Family Dipodid^e. — This and the four following families are 

 embraced in the section Myomorpha. In the Dipodidce (Jerboas) 

 remains of the existing Alactaga jaculus occur in the Pleistocene of 

 Europe, and those of Zapus {Jaculus) hudsonianus in the correspond- 

 ing beds of North America ; while a species of Platycercomys has 

 been recorded from the Pleistocene of Northern Asia. The so-called 

 Dipoides of the Tertiary of Wiirtemberg is probably a Ckalicomys. 

 Family Geomyid^:. — The American Pouched-Rats, in which the 



1 3 



cheek-teeth are Pm. -, M. -, are represented by species of the type 



genus Geomys in the Pleistocene and Pliocene of North America, 

 and also by one existing species of Thomomys in the Pliocene of 

 Oregon. In the Miocene of the United States two extinct genera 

 are also met with, of which Entoptychus is allied to Thomomys, and 

 has rootless molars and broad incisors ; while Pleurolichus has rooted 

 molars and the incisors without grooves, like those of the allied 

 existing genus Heteromys. 



Family Spalacid^e. — This Old World family comprises burrow- 

 ing Rodents, with large incisors, and rooted molars having re-enter- 

 ing enamel-folds. The only fossil representative is a Rhizomys, 

 from the Siwalik Hills of India, which appears to be closely allied 

 to the living forms ; all of which are characterised by the absence of 

 premolars. 



Family Murid^e. — The Muridcz form by far the largest family 



