722 MONOGRAPHS OT NORTH AMERICAN RODENTIA. 



trustworthy character of white nose and ears in var. niger, though open to 

 many exceptions, is the one mainly to be relied upon in distinguishing the 

 two varieties. There is apparently a gradual and quite marked increase in 

 size southward in these two forms, so that size alone tails to he distinctive, 

 especially in the central portion of the. Atlantic States, or wherever their 

 habitats meet. 



Var. ludovicianus is generally readily separable by its coloration from 

 either var. cinereus or var. niger. The rusty-bellied style of var. cinereus, 

 however, is often quite inseparable from specimens of var. ludovicianus from 

 northern localities. Particularly is this the case when Pennsylvania speci- 

 mens of the rufous type are compared with Ohio examples of var. ludovici- 

 anus. There are, for instance, two specimens before me without labels, and 

 hence from unknown localities, which I cannot positively refer to one of these 

 varieties rather than to the other. There is little, if any, difference in size 

 between vars. cinereus and ludovicianus, although the latter is pretty constantly 

 appreciably smaller than var. niger, as shown by a comparison of the forego- 

 ing diagnoses of the three varieties. The external measurements are, unfor- 

 tunately, nearly all from skins, and are hence not very trustworthy. Taking 

 the skulls as a standard of comparison, var. niger averages appreciably the 

 larger, four skulls of niger averaging 2.75 against 2.58 in both vars. ludovici- 

 anus (eight skulls) and cinereus (three skulls). 



Geographical variation. — In varieties cinereus and niger, the only 

 appreciable feature of geographical variation is the before-noted gradual 

 increase in size southward. A large series of specimens, from numerous 

 localities, might show that other variations correlate with differences of 

 locality. My material illustrative of these two forms is unsatisfactorily 

 scanty. Var. ludovicianus, on the other hand (thanks to more abundant 

 material), shows strongly marked geographical variations in color, but I tail 

 to notice any well-marked geographical variation in size. Judging from the 

 skulls, the northern specimens are rather the larger, yet the largest skull of 

 the series is from Brookhaven, Miss. The measurements taken from the 

 skins show also a rather greater size northward, the largest specimens 

 being from Ohio and Fort Randall, Dakota, if length of body be taken 

 as the standard; if, on the other hand, the length of the hind foot be 

 adopted, in consequence of being less susceptible to variation resulting from 

 taxidermy, the southern specimens average nearly or quite as Lirge as the 



