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 olf, 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 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 zygomatic 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 hand to 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 , 
