238 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
teeth, the internal saliencies very acute; of the upper series, the internal sali- 
encies obtuse and the internal reéntrances shallow; of the lower series, the 
external saliencies obtuse and the external reéntrances 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 Ist 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 Arvicole; form very stout and 
compact; colors usually variegated and often changing with season, as im 
many other Arctic* mammals; pelage very long and thick, cold-proof; snout 
blunt, hairy except the nasal papille; 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 
