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



In the lower jaw, the anterior molar occupies about one half of the entire grinding surface. 

 There are five salient angles on the inside, and four on the outside, besides the terminal anterior 

 rounded loop, which is slightly angular on either side, thus presenting an obsolete salient angle 

 in addition to those described. It rounds off, however, obliquely, outwards, the direction of its 

 axis being the same. The dentine in this loop is continuous with that of the adjacent salient 

 angles on either side, so that there are but seven dentine islands completely enclosed by enamel ; 

 one anterior, one posterior, three internal, and two external. 



This skull differs from ?//9 from Middleboro, Mass., in being narrower and considerably 

 more elongated ; this excess- of length is chiefly in the cranium ; the occiput projects unusually 

 far behind the interparietals: The zygornatic arch extends further behind. The muzzles are 

 of about equal length. There is little appreciable difference in the outlines of the molars. The 

 interparietal is truncate externally, instead of extending outward to a point ; there is, however, 

 a point produced outward from the posterior corner of the truncation. 



Variety longipilis Specimens from West Northfield, Illinois, and Racine exhibit decided dif 

 ferences in many respects, but I am not prepared to consider them as distinct species, especially 

 as all are either winter or early spring skins, and as such naturally different in color and texture 

 of fur. I have not seen any, however, from other localities possessing the same characteristics. 

 Further examinations must show their real position. They are among the largest of the United 

 States species, and eminently characterized by the great length, fullness, and softness of the fur, 

 in these respects considerably exceeding any other skins in the Smithsonian collection. The 

 fur is so close that the skin cannot be seen between the hairs when these are blown apart. The 

 shortest hairs of the back measure half an inch ; the long bristly ones three-quarters of an inch. 

 The prevaling color of the upper parts is a very dark brown, almost black, without any of 

 the reddish or yellowish impression which most of our other species convey. There is a slight 

 shade of yellowish brown at the tips of the hairs, especially towards the sides. The under 

 parts are of a plumbeous ashy, without any tinge of rusty, or pure white. There is no line of 

 demarcation between the colors of the back and belly, one passing insensibly into the other. 

 The upper surfaces of the feet are plumbeous, (not brown nor reddish.) The under half of the 

 tail is hoary plumbeous, rather lighter than the belly, and quite distinctly defined. 

 The feet are moderately long ; the soles with six tubercles, the posterior very large. 

 The essential characters of this animal are found in the very long and soft fur ; and the 

 nearly black color of the back and sides, without any of the prevalent reddish brown of other 

 specimens. The under parts, too, are of an unusually pure plumbeous ash ; the tail very dis 

 tinctly bicolor. If, as I think very probable, further investigation will prove this to be an 

 undescribed species, the name of longipilis may be very appropriately applied to it. 



In the synonomy of this species I have omitted any attempt at introducing any of the many 

 species of later authors which will, in all probability, find their place here. The only question 

 that needs any special discussion here is in reference to the name A. pennsylvanica of Ord. 

 This was based by him on the "meadow mouse" of Wilson, (Ornithology, VI; pi. 1, f. 3,) 

 and first named in the second American edition of Guthrie's Geography, II, 1815, 292. I have 

 not the description of Ord at handito see how fully it agrees with his A. riparius, but the figure 

 of Wilson amounts to nothing. In the account of A. pennsylvanica by Harlan, (F. Am. p. 144,) 

 the animal is described as brownish fawn above, grayish white beneath, eyes very small, ears 

 short and round. Length of head and body, four inches ; of tail, three-quarters of an inch. 

 Teats of the female in the groin. Now this description applies quite well to A. pinetorum, 



