212 MONOGRAPHS OF NORTH AMERICAN RODENT1A. 



These colors fade on the si<les, without any tangible dividing line, into 

 the peculiar shade <>!* the whole under parts. In an average case, the belly 

 shows a background of plumbeous, strongly washed over with a dirty cin- 

 namon, or muddy rust color. In the darkest-colored individuals, the under 

 parts are deep hoary-plumbeous, with the tips of most of the hairs barely 

 touched with muddy ; this peculiar shade, so different from the clear hoary- 

 plumbeous of riparius, &c, being a strong mark of the subgenus, and only 

 very exceptionally wanting. In the lightest-colored specimens, on the other 

 hand, the under parts are so strongly invaded with the muddy cinnamon that 

 the plumbeous bases of the hairs are scarcely visible, the dirt-color being 

 continuous, especially along the sides, and so bright as to approach a fawn- 

 color or tawny-brown. 



The tail is almost always distinctly bicolor, and it shares the colors of 

 the upper and under parts of the body respectively. 



The type of the species (No. 2249, Mus. Smiths.) is a rather unusually 

 dark specimen, especially underneath, being, as Professor Baird lias remarked, 

 one of the few in which the cinnamon tips of the hairs are inappreciable. 

 Other specimens, however, received from Major LeConte as typical of his 

 species, have the muddy wash very distinct. 



The Louisiana specimen enumerated by Professor Baird (No -nrrr, Cal- 

 casieu Pass, G. Wurdemann) is typical austerus, and extends the known range 

 of this form. A Kansas example (No. 4218, Neosho Falls, B. F. Goss) is 

 likewise pure austerus. Another Kansas specimen (No. 3306, Doniphan 

 County, E. Palmer) leans rather over against var. curtatus in the shortness 

 of its tail, though it is typical austerus in other respects. A Platte River 

 specimen (No. 30D4) is identical with the type of "haydeni". The exact 

 state of the case regarding this last is given beyond; here it only remains to 

 examine the other nominal species that has been referred to austerus. 



The type and only known specimen of "cinnamomea" (No. tttz, 

 Pembina, Minn.) is, as Professor Baird says, exactly like austerus in external 

 characters The points of difference, if any, lie in the skull and teeth; and 

 we have the data to show that the slight differences observable in these 

 respects arc quite within the limits of individual variation. On coming into 

 our hands, the skull lacked zygomata; but the zygomatic width is stated by 

 Professor Baird to have been 0.5(J, which, with a length of 1.12, gives a 

 proportion of just 100 : 50, which is a little greater length for breadth than 



