244 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



acters closely conforming to t he last named. Palatal shelf as in My odes i 

 zygomata much less laminar than in Myodes (nearly as in the larger Arvicolee). 

 Nasals and nasal branch of premaxillaries subequal; both very short, ending 

 opposite anterior root of zygomata. Orbits less encroached upon by the 

 cranial dome than in Myodes, but having a prominent pointed process 

 for muscular attachments. Superior incisors as in Myodes; and likewise as 

 in that genus, roots of under incisors stopping opposite back under molars. 

 Molar series long and strongly convergent anteriorly, as in Myodes, but the 

 pattern of the crowns entirely different and strongly arvicoline, as follows: — 

 Front upper molar of seven (five in Myodes) prisms: an anterior transverse 

 spherical triangle, three internal lateral triangles, two external lateral trian- 

 gles, and a (small, supplementary) postero-external loop. Middle upper molar 

 of six prisms (four in Myodes): an anterior transverse loop, two external 

 lateral triangles, two interior lateral triangles, and a (small, supplementary) 

 postero-external loop. Back upper molar of six prisms: an anterior trans- 

 verse loop, two external and two internal lateral triangles, and a posterior 

 trefoil, or V or U. Front under molar of nine prisms (five in Myodes): an 

 anterior trefoil, three external lateral triangles, four internal closed triangles, 

 and a posterior transverse loop. Middle and back under molar each of live 

 (or five and a half) prisms : an antero-external triangle (with a more or less 

 evident anterior lobe abutting against the back loop of the antecedent tooth), 

 two internal lateral triangles, one external lateral triangle, and a posterior 

 transverse loop. All the lateral triangles of all the teeth alternating. External 

 form stoutest and most compact in the. subfamily; limbs the shortest; no 

 external ears ; muffle completely hairy except the very papillae ; pelage dense 

 and woolly; feet short, stout, both fore and hind completely furry both above 

 and below, the longer hairs reaching usually far beyond the ends of the claws ; 

 pollex obsolete, with abortive nail; third and fourth digits much longer than 

 second and fifth, their claws periodically hypertrophied and quasi-duplicated 

 by an enormous growth of corneous substance on their under surface; hind 

 claws ordinary; tail to end of vertebras shorter than the hind foot, but copi- 

 ously comous, the terminal pencil usually longer than the vertebral moiety. 

 Coloration subject to periodical changes: dark and variegated in summer, 

 snow-white in winter. 



As will be seen by the above, the cranial characters and those of the 

 incisive dentition are very nearly the same as in Myodes, but that the pattern 



