735 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
Some of these have well-developed ear-tufts, while others are entirely with- 
out them. One specimen has the posterior half of the dorsal surface dusky 
yellowish-brown, while the rest of the body is dusky, and the tail is nearly 
black. There is also a wholly black specimen in the collection from the 
Apache Mountains, Arizona. 
This species is considerably smaller than S. fossor, with a rather stouter 
body and shorter tail. One specimen (No. 9549), from the mountains north 
of Santa Fé, N. Mex., has the general color above of S. fossor, showing only a 
very narrow yellowish-brown dorsal stripe. It has, however, a broad, black- 
ish, lateral line, and the under side of the tail is wholly white These are 
features that serve at once to distinguish the two species, aside from the dif- 
ference of size, relative length of the tail, ete. From all the gray phases of 
S. carolinensis, it is distinguishable by the absence of the subterminal fulvous 
suffusion of the pelage above, which is constantly present in S. carolinensis ; 
by the absence of a white ear-patch ; by the coloration of the tail; by the much 
larger size of the ears; and by other quite obvious differences. 
In respect to the skulls of the gray North American Sciwri possessing - 
two premolars, namely, S. fossor, S. aberti, and S. carolinensis, the skull of 
S. carolinensis is distinguishable by its narrow elongate form, its smaller size 
(especially as compared with S. fossor), and particularly by the excessive 
narrowness of the nasal portion and consequently narrower nasals and upper 
incisors. The chief difference between the skulls of S. aberti and S. fossor 
is the much larger size of the skull in S. fossor. Two skulls of S. aberti, 
both fully adult (one of them very old), have an average length of 2.30 against 
an average length of 2.60 in nine skulls of S. fossor, the smallest of which 
(rather young) has a length of 252. The first premolar seems to be rela- 
tively larger in S. aberti than in either S. fossor or S. carolinensis. 
The geographical range of S. aberti, as shown by the specimens in the 
collection of the National Museum, extends from the mountains of Central 
Colorado southward to the Apache Mountains of Arizona. Its occurrence 
does not appear to have been reported from any locality outside of Colorado, 
New Mexico, and Arizona. Its range is hence apparently separated by quite 
a wide interval from that of S. fossor, neither species having been yet 
reported from either Utah or Nevada. 
The S. castanonotus of Baird, formerly regarded by Professor Baird as 
distinct from S. aberti, proves to be not separable, even as a geographical 
