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U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



with an internal concavity. The difference of two or three external triangles to the*anterior 

 lower molar depends only on the greater or less indentation of the anterior re-entrant angle, 

 and may vary in the same species. 1 



It is to this group thai by far the greater number of the American species belong, especially 

 those of largest size. While new species remain to be discovered, it is highly probable that 

 many of those indicated as distinct will, on a critical examination, prove to be the same. The 

 law of specific character in our American species has not yet been fully ascertained, but it is 

 beyond a doubt that species may vary considerably (even if within fixed limits) in dimensions 

 in the size and amount of hair on the ears, the length of the tail, and the color of the fur. 



The species of this group described in the following pages are : 



EASTERN NORTU AMERICA. 



Arvicola riparia, 



" longipilis, 

 breweri, 

 rufidorsum. 



WEST COAST AND KOCKY MOUNTAINS. 



Arvicola townsendii, 

 montana, 

 longirostris, 

 occidentalis, 

 califomica, 

 edax, 

 modesta. 



ARVICOLA. B. CHILOTUS, Baird. 



Ears small, orbicular, the marginal portion or helix incurved all round, bounding a distinct fossa innominata ; the upper 

 and lower roots so close together as to be confluent anteriorly, and thus enclosing the meatus entirely in the anterior rim, 

 however low ; surfaces of the ear almost naked ; tubercles of soles fine, as in A. pindorum ; fore feet but little more than 

 half the hinder ; fore claws not longest ; tail about one-third the head and body. Second upper molar, with one internal 

 triangle ; third, with one external ; anterior lower molar with three internal closed triangles and two or three external. 



Size very small. Fur close, short. Ears short, naked, with scattered short hairs. Feet 

 rather small and weak ; anterior little more than half the posterior ; claws not longer. Tail 

 longer than the head. 



Skull rather broad. Nasal branch of intermaxillary as long as the nasal bone, and not 

 reaching to the inter-orbital region. Interparietal bone subtruncate laterally. Occipital 

 foramen round ; as wide as high. Second upper molar, with four closed triangles. Posterior 

 upper molar, with one anterior triangle ; one lateral on either side, and a posterior Y or trefoil. 

 The anterior lower molar has one posterior triangle, three internal and three external triangles, 

 with an anterior trefoil loop. Middle lower molar with five triangles, the opposite salient 

 angles alternating. 



I know of but one species of this group, the A. oregona of Audubon and Bachman. 



1 Arvicola agrestis, from Sweden, in the character of its skull and teeth, belongs strictly to the group having A. riparius for 

 the type. The anterior lower molar has three lateral triangles on each side. 



