94 MICROTIN^ 



2. Cheek-teeth rootless and persistently growing. Bony 

 palate normal; usually boldly sculptured in adults. 

 Mandible with m^ noticeably displaced by shaft of 

 incisor. Auditory bullae with the stapedial artery 

 enclosed in a bony tube. 

 a. Inguinal mammae always present and functional; 

 /»! with its fourth and fifth triangles, according to 

 the genus, either constantly closed off from, or con- 

 stantly open to each other. 

 1. Enamel pattern of m^ and m^ simphfied; m? with 

 three salient angles on each side; rftj with three 

 outer and five inner saUent angles, its third 

 outer fold more or less reduced, with three 

 closed triangles only and all parts in front of the 

 third triangle merged in the anterior loop. 



Skull with the temporal ridges fused in the 

 interorbital region when adult. 



External form always more or less modified 

 for fossorial or for aquatic habits. 



a. Size medium to large. 



Skull massive, strongly ridged and angular 

 when adult ; post-orbital crests of squamosals 

 well developed ; squamosals tending to approach 

 each other anteriorly with advancing age. 

 Auditory bullae small, with a shght develop- 

 ment of spongy bone tissue within. 



External form modified sUghtly for aquatic 

 habits, but in some species rather more 

 markedly for fossorial life. Hind-claws slightly 

 longer than fore-claws ; palms and soles naked ; 

 hands and feet sUghtly fringed laterally. Tail 

 about half length of head and body. Mammae 

 2—2=8 Arvicola. 



b. Size small or medium. 



Skull less strongly built; post-orbital crests 

 small but salient ; squamosals widely separated 

 anteriorly. Auditory bullae large, their walls 

 strengthened by dense spongy bone tissue; 

 mastoid portion shghtly inflated. 



External form modified for fossorial life ; fur 

 long and soft ; palms and soles densely haired, 

 the pads concealed; claws unusually long. 

 Tail short, about one-fourth length of head 

 and body. Mammae 3(2) - 2 = 10(8). Phaiomys. 

 ~. Enamel pattern of mj less reduced, its third outer 

 infold always well developed, and one or more 

 of the dentinal spaces in front of the third closed 

 triangle always separated from the anterior 

 loop. 

 a. nil with only three closed triangles, the fourth 

 and fifth triangles confluent with each other 

 and substantially closed off from the parts 

 anterior to them. Re-entrant folds of cheek- 

 teeth partly filled with cement, 

 a. Cranial and external characters modified for 

 fossorial life. 



