MURID^— ARVICOLlNiE— MYODES. 



239 



each other, the 1st much sliorter, the 5tli shorter still ; soles usually densely 

 furry to the claws, but sometimes showing the under surface of the toes ; * 

 plantar tubercles naked. Tail very short, its vertel)rae shorter or not longer 

 than the sole, leporine, stout, densely hairy throughout, with a copious ter- 

 minal pencil, often longer than the vertebrae. 



The foregoing diagnosis, so drawn as to exclude Cuniculvs, is based upon 

 Mus lemtnus of Linnaeus, and indicates a perfectly natural generic group of 

 ,ArvicolineB. From Arvicola, in any of its subgeneric phases, Myodes is prom- 

 inently distinguished by external form as well as by cranial and dental char- 

 acters. The general clumsy shape, very convex-obtuse head, short rabbit-like 

 tail, short ears, small furry feet, elongated fossorial claws, and moUipilose 

 pelage, are associated, in the skull, with breadth and massiveness, laminar 

 expansion of the zygomata, and a peculiar shape of the palate ; and, in the 

 teeth, with stoppage of the root of the under incisor opposite the last 

 molar, a quill-like beveling of the upper incisors, and a particular pattern of 

 the molar crowns. All the points of external form that mark off Myodes from 

 Arvicola likewise separate it from Synaptomys ; these two agreeing almost 

 precisely in cranial and dental characters. The plane, instead of grooved, 

 upper incisors are distinctive of Myodes, and so is, to less degree, a slight 

 difference in the middle lower molar (vide descriptions). With Myodes, 

 Cuniculus is generally associated generically ; but we wish to particularly 

 signalize -the fact that they are perfectly distinct genera. Although both of 

 them are "lemmings", so called; and although they do agree in general 

 external tournure, yet they present differences fully on a par with those made 

 the basis of generic distinctions in other cases. How great these differences 

 are may be inferred, by one not acquainted with the animals, from the fact that 

 Lilljeborg, who adopts only four genera for the whole subfamily, keeps the 

 two apart, his genera being Fiber, Arvicola, Cuniculus, and Myodes. The 

 comparative diagnoses are fully given farther on ; here we will only add, that 

 in Myodes the external ears, though small, are perfect, while Cuniculus has 

 no external ears ; that in Myodes, though the fore claws are lengthened and 

 "fossorial", they never show the extraordinary development seen in Cuniculus; 

 that the rudimentary pollex of Myodes bears a large ligulate nail, only faintly 

 indicated in Cuniculus by an abortive thumb and claw ; and, finally, that with 

 most cranial characters in common, the pattern of the molars is very different 

 in the two genera. Unlike Myodes, Cuni 'us turns white in winter. 



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