44 



MAMMALOGY. 



" The specimens were obtained near Callao, Peru. Their nests 

 were found in the fences, built of adobes or sun-dried bricks, and con- 

 sisted of fine grass, feathers, &c." 



4. Genus ARVICOLA, Lacepede (1803).— The Field Mouse. 



1. Arvicola occidentalis, PeaU. — The Western Field Mouse. 



Arvicola occidentalis, Peale, Zool. Exp. Exp. Mamm. p. 45 (1st ed. 1848). 



Atlas, Mammalogy, Plate VIII, Fig. 2, Adult. 



Pilis molissimis et tenuissimis, extremitatihus superior ihus rufis sine ulla 

 nigra admistione. Aurihus suh pilis occidtis, cauda suhcompressa, 

 supra et suhtus concolore mfa. Long. 6 unc. 



Form. — Head large, blunt ; upper incisors not grooved ; eyes small ; 

 ears rather large, but nearly concealed by the long hair ; antitragus 

 rather small, semicircular. Thumb of fore foot rudimentary, with 

 a compressed blunt nail ; third and fourth toes of the fore foot 

 nearly equal ; fifth, shorter than the second. Hind foot with the 

 thumb or first toe about half as long as the second ; third and 

 fourth, very nearly equal ; fifth, rather more than half the length 

 of the fourth. Tail slightly compressed laterally and covered 

 with short hair; bristles of the muzzle rather numerous and long. 

 Ears covered with hair on their outside only. Feet covered with 

 short hair. Hair, especially on the upper parts, very fine and soft. 



Dentition. — In the upper jaw, the first molar presents five triangles, 

 one anterior, two exterior, and two interior ; the second presents four 

 triangles, one anterior, two exterior, and one interior, with a posterior 

 rounded lobe ; the third pi'esents five triangles, one anterior, two 

 exterior, very small, two interior, and ends in a trefle or three-lobed 

 figure, the exterior lobe of which is nearly obliterated, and leaves a 

 plane space from the end of the tooth to the posterior triangle. The 

 line of teeth in the upper jaw exhibits, externally, nine salient and 

 nine re-entering angles, the two last of each, small ; internally, ten 



