ISO MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



"longirostris" 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 

 (No. t5Tt) 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 interparietal acute instead of subtruncate. 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 Arvicolce". 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. fflf), 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 arc gray-brown, 

 darker along the middle of the back, especially toward the tail. A hay 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. 



