238 MONOGRAPHS OF NORTH AMERICAN KODENTIA. 



teeth, the internal saliencies very acute; of the upper series, the internal sali- 

 encies obtuse and the internal reentrances shallow; of the lower series, the 

 external saliencies obtuse and the external reentrances shallow; first upper 

 molar of 5 triangles, 1 anterior, 2 interno-lateral, and 2 externo-lateral, these 

 alternating, the first lateral one. being internal; middle upper molar of 4 tri- 

 angles, the first 2 transverse, 3d interno-lateral, 4th externo-lateral ; back 

 upper molar of 4 triangles, or rather crescents, all transverse; front lower 

 molar with 5 triangles; an anterior irregular trefoil, a lateral posterior loop, 

 1 external and 2 internal lateral triangles; middle lower molar like front lower 

 molar, but an interno-anterior triangle replacing the trefoil; back lower molar 

 with 4 triangles, the 2 posterior entirely transverse and approximated at their 

 outer ends, the other 2 lateral, alternating, the 1st being antero-internal. 

 (Thus, the front molars, both upper and lower, are 5-prismatic, with as many 

 inclosed dentine islands on their crowns; the back molars, both upper and 

 lower, are 4-prismatic, with as many inclosed dentine islands on their crowns; 

 the middle upper like both back ones, 4-prismatic ; the middle lower, like 

 both front ones, 5-prismatic ; and this 5-prismatic middle lower molar is the 

 only diagnostic one as compared with Synaptomys, in which the same tooth 

 has only four prisms.) Upper incisors ungrooved, but much beveled off later- 

 ally, and appearing like tubes of enamel through deficiency of dentine behind, 

 where they begin to come to an edge ; about as wide as deep, and much 

 curved. Under incisors remarkable in that their roots do not reach beside 

 and behind the last molar up the condylar ramus, but stop abruptly in front 

 of the last molar. 



External chars. — Size of the larger Arvicolce ; form very stout and 

 compact; colors usually variegated and often changing with season, as in 

 many other Arctic mammals ; pelage very long and thick, cold-proof; snout 

 blunt, hairy except the nasal papillae ; external ears, though well formed, 

 small, at most never coming to the surface of the fur; fore feet large, with 

 the claws about equaling the hinder without the claws; thumb obsolete, 

 with a large ligulate claw ; all the other claws very long and fossorial, exceed- 

 ing the hinder, but lacking the peculiar development seen in Cuniculus; 3d 

 claw longest, 4th but little shorter, tip of 2d reaching base of 3d, 4th much 

 shorter; palms sometimes naked, sometimes furry to the claws; when naked, 

 thin, tuberculate. Hind feet short, only exceeding the fore by the. length of 

 their claws; their claws moderate; 3 middle toes longest and about equaling 



