540 



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



in some specimens they predominate along the back. The sides are rather paler. The belly 

 is a pale cinnamon, brighter than that of the upper parts ; the colors of the sides and belly 

 melt quite insensibly into each other. The basal lead color of the under fur is generally seen 

 to a greater or less extent. The feet and upper part of the tail are colored somewhat like the 

 back, or cinnamon brown lined equally with black, the tail darkest, sometimes nearly uniform 

 blackish brown, especially towards the tip ; the under part of the tail like the belly. 



There does not appear to be much variation in the colors of this species, except that in a 

 midsummer specimen the colors are a little brighter and darker cinnamon. Winter specimens 

 are paler. A skin from St. Louis has the longest fur of all the skins in the collection, and has 

 the cinnamon color of the belly very distinct. 



1 From West Northfield * From Racine. 



1999. The skull of this species is of average breadth among Arvicolae, its dimensions being 

 (in No 1999) 1.10 inch by 63, or as 100 : 57. The cerebral portion is rather short, the narrow 

 ing of the interorbital space commencing about the middle of the axial line of the skull, or even 

 a little behind it. The nasal branches of the intermaxillary extend further back on the fore 

 head than the nasal bone, and in some cases pass the line of the lachrymals. The line ot 

 molars is short in the upper jaw, less than one-fourth the length of the skull. The distance 

 from the upper molars to the posterior face of the incisors is about one-third that from the 

 incisors to the occiput, or a little more in some specimens. 



The posterior upper molar is about equal to the middle one, and shorter than the anterior ; 

 it is composed of one anterior triangle, one exterior, one interior, and one posterior loop or 

 hook. This loop is somewhat V-shaped, connected with the pedicel of the interior triangles by 

 its outer leg ; it is rather acute behind, and both legs are slightly concave both on the inner face 

 of the tooth, and the outer. There are thus but three salient angles on either side, including 

 the posterior loop. 



The anterior lower molar is less than half as long as the other two. It presents five internal 

 salient angles and four external, besides the anterior loop which is angular on the side and may 

 be considered as forming an additional angle, especially externally. There are, however, of com 

 pletely closed triangles, one posterior, two internal and one external ; the anterior lobe has two 

 salient angles on either side, and an anterior sub-angular loop, the dentine in all broadly 

 communicating. 



In one specimen, 1997., the skull of which is larger than others, the teeth are wider ; the first 



