NATURAL SCIENCES OF PHILADELPHIA. 327 



extension not precisely known. Specimens examined from the 

 Yellowstone, Powder, Niobrara, Platte, and Arkansaw Rivers ; 

 from various localities in Texas, and nearly throughout New 

 Mexico and Arizona; from Sonora, Durango, and Coahuila, Mexico. 



Chars. Larger : rather over than under 4 inches in length of 

 head and body, with (comparatively) stout shape, small ears, 

 short limbs, and short tail. Tail vertebras one inch (more or less) 

 longer than the head and bod}', bearing a proportion of about 

 (rather less than more) 1.25 to 1.00. Coloration light ; upper parts 

 nearly uniform tawny-brown of the shade peculiar to the genus, 

 darkened a little with mouse-brown on a dorsal area. 



This form of the genus appears to have been first noted by Dr. 

 Woodhouse, in 1853, all the other names being referable to the 

 other. His specimens were from El Paso. The known limits of 

 its dispersion were enlarged in 1857 to include the region of the 

 Platte; while specimens still more recently examined show that 

 it extends northward to the Yellowstone at least, further east in 

 Dakota, Nebraska, and Kansas than was formerly supposed, and 

 even reaches to Arkansas, where specimens were lately procured 

 at Fort Cobb. 



