INTERRELATIONSHIPS OF GENERA 



67 



and incisor teeth are normal. The cheek-teeth are endowed 

 with persistent growth, and have the enamel normally and 

 sometimes conspicuously differentiated, thick enamel forming 

 the concave sides of the salient angles, thin enamel forming 

 their convex sides ; cement is present in the infolds. In pattern 

 m^, tn^, mg, and m^, are normal, with or without clear traces of 

 cusp n; m^ has from three to five salient angles on each side; 

 mj consists of a posterior loop, usually followed by five 

 substantially closed triangles and terminated by an anterior 

 loop ; in some forms the fifth triangle (fourth inner angle) is 

 confluent with the anterior looj) ; in others, in which the 

 anterior loop is extraordinarily complex, additional triangles 

 may be closed off from the base of the loop so that the number 

 of the closed triangles rises to six, seven or even eight. 



Chilotus, represented by a few species in North America 

 and Asia, is very closely related to Microtus, but it is somewhat 



Fig. 41. — Cheek-teeth of Microtus clarJcei Hinton. 

 Crown views : a. right upper, b. left lower molars. 



modified for more fossorial habits, or j^ossibly for creeping under 

 logs or stones. Its skull is curiously depressed, and the mastoid 

 portions of the auditory buUiB are distinctly inflated. The fur 

 is short and dense, without any admixture of stiff hairs. The 

 ears are rather small, and the plantar tubercles are reduced to 

 five. In all other respects it agrees with typical Microtus. 



Lasiopodomys is a small but rather remarkable Asiatic 

 genus. Closely related to Microtus, as is shown by the almost 

 typical skull and teeth, it is considerably specialized externally 

 for fossorial habits, somewhat in the manner of Lemmus or 

 Phaiomys. The fur has become soft and fine ; the ears are short, 

 scarcely appearing above the fur, and are nearly naked. The fore- 

 claws are considerably lengthened, and the thumb is armed with 

 a sharp claw instead of the usual flattened nail. The soles of 

 the hind-feet are densely haired, and although six pads are 

 present, the two posterior are very small, placed low down, and 

 completely hidden beneath the hair. The tail is short and 

 densely clothed. The skull is broad and rather flat, with normal 



