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244 



MONOORAPlia OF NORTH AMERICAN RODENTIA. 



ncters closely canforming to the last tmincil. Palatal shelf os in Mymles; 

 zygomata much less laminar than in Myodfs (nearly as in the larger ^n'Jfo/<p). 

 Nasals ond nasal branch of premaxillarios suJM'quttl; botii very siiort, ending 

 opposite anterior root of zyg(Hiiata. Orbits less encroached upon by the 

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

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

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

 Jilolar series long and strongly convergent anteriorly, as in Mi/odes, but the 

 j)attern 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, tA^o interior lateral triangles, and a (small, supplementary) 

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

 verse loop, two external and two internal lateral triangles, and a ])osterior 

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

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

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

 (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 sultfamily; limbs the shortest; no 

 external ears; muffle com|)letely hairy exce|)t Mie very papilltc; pelage den.se 

 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 ; 

 poUex 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 vertebrse shorter than the hind foot, but copi- 

 ously comous, the termiiuil 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 tiie above, the cranial characters and those of the 

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



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