180 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
‘Jongirostris” is only a slight increase in the forward tilting they often show ; 
in fact, several eastern skulls differ more among each other than one of them 
2) does from longirostris. There is nothing peculiar in the dentition 
of the latter. 
A. breweri is stated to be narrower behind the zygomatic arches, and to 
have the interparictal acute instead of subtruneate. At a particular point 
across the back of the skull, it measures 0.40; several other skulls measured 
at the same point give dimensions equal to, greater than, and less than this. 
No two specimens in the whole series of skulls are exactly alike as to the 
lateral corner of the interparietal; sometimes it is an edge instead of a corner, 
sometimes obtuse, sometimes acute; and when thus attenuated, as it fre- 
quently is, the sharp spur may be turned backward, forward, or neither way. 
Our general description of the skull and dentition of riparius is made 
elsewhere; here we will merely inquire whether or not any of the dental 
peculiarities ascribed to the several nominal species will hold good. 
A peculiarity of the last upper molar of A. californicus is stated to be a 
short lobe that the posterior crescent sends outward near its posterior portion ; 
but we cannot agree with the author that this is a “character rarely observed 
among American Arvicole”. It is, in fact, not an unusual condition of the 
several inextricably-graduated variations of this last upper molar; we have 
seen it frequently, and, in one specimen we have just picked up (No. 2527), the 
variation is carried to such extreme that the back part of what ought to have 
been the convexity of the crescent is made concave. The ‘supplementary 
internal lobe” of the same tooth of “occidentalis” is a very common feature 
in eastern and other skulls. Not to prolong this inquiry further, we may state 
that we have satisfied ourselves by personal examination that no one of the 
supposed western species possesses any dental features not matched by 
examples of eastern riparius. 
We will next turn to the matter of color. - 
As already stated, there is no appreciable variation in color in the 
nineteen Philadelphia skins, unless it is that some of them have the tail 
a trifle more decidedly bicolor than others; but in none is the definition 
of the lighter and darker surfaces very distinct. They are gray-brown, 
darker along the middle of the back, especially toward the tail. A bay tinge 
or reddish-brown is scarcely appreciable ; so that, without being at all ‘“black- 
ish” in general hue, the shade is darker than that of rufescent specimens. 
