20 



Obs. The limits I would set to Armcola may be gathered from 

 the foregoing, though doubtless some synonyms, and, perhaps, 

 some tenable subgeneric names, of old world forms, have escaped 

 me. Without, therefore, writing out the full diagnosis, I will 

 simply mention characters which separate Arvicola as I accept it 

 from the other North American ArvicoUnse : — 



Molars normally rootless, perennial (c/. Evotomys). Folds of 

 enamel fusing at the re-entrances (cf. Evofomys). Molars viewed 

 from above about equally serrate on either side (cf. 3Iyodes, 

 Synaptomys). Upper molars all subequal in length and breadth 

 {cf. Myodes, Synaptomys). Middle lower molar of 4-5 prisms, 

 the last a transverse loop, the rest alternating lateral triangles {cf. 

 Evotomys). Back lower molar of not more than 3 prisms seriatim 

 {cf. Synajjtomys, Myodes)., each making a transverse loop. Gene- 

 ral molar pattern, as to number of prisms to a tooth, as shared 

 by most Arvicolinse {cf. Guniculus). Tipper incisors plane {cf 

 Synaptomys). Roots of under incisors running up the condylar 

 process forming a ridge that only subsides near the notch between 

 condyle and descending process {cf. Synaptomys., 3Iyode.s, Ciini- 

 culus). Palate not ending behind in a flat straight-edged shelf 

 {cf. Evotomys, llyodes, Guniculus). Coronoid process as high as 

 condyle {cf. Evotomys). Ears moderately developed {cf. Myodes, 

 Guniculus), but not overtopping the fur {cf. Evotomys). Tail 

 longer than foot {cf Myode>^, Guniculus). Fore claws of normal 

 size and growth {cf. Guniculus). The species do not turn white 

 in winter ( cf. Guniculus). Many other details could be adduced. 

 The North American species fall in four sections or subgenera, 

 as established by Baird. I use Myonomes, Raf., for the section 

 for whicli Baird employed Hemiotomys, Selys, as none of our sec- 

 tions seem exactly the same as an^^ one of Europe. Pedomys and 

 Pitymys are like each other in skull and dentition, but offer ex- 

 ternal peculiarities. The sections may be thus analyzed: — 



A. Myonomes. Back upper molar with 3 external triangles and a pos- 



terior crescent. Middle upper molar with 2 internal triangles. Front 

 lower molar with 3 internal and 2 or 3 external lateral triangles. 

 Ears iinrimmed in front. Soles 6-tuberculate. Fore claws not longer 

 than the hinder ones. Tail about ^ the length of head and body, or 

 more. Pelage ordinary. Size maximum and medium. 



B. Chilotus. Back upper molar with only 1 external triangle and a pos- 



terior trefoil. Middle upper molar with 1 internal triangle. Front 

 lower molar with 3 internal and 2 or 3 external triangles (as in 



