174 Field Museum of Natural History — Zoology, Vol. XI. 



Total length 6.62 to 7 in. (169 to 179 mm.); tail vertebras 2.75 to 3 in. 

 (70 to 77 mm.). White-footed Mouse. 



Peromyscus leucopus, p. 184. 



Total length 5.50 to 6.50 in. (140 to 165 mm.); tail vertebrae less than 



2.75 in. (70 mm.). Prairie White-footed Mouse. 



Peromyscus maniculatus bairdi, p. 190. 



Total length about 5 in. (128 mm.); tail vertebrae from 2 to 2.50 in. (50 



to 65 mm.); back brown; sides grayish buff; a distinct longitudinal 



groove on front of upper incisors near outer edge. Not as yet recorded 



from within our Hmits but may occur. Dyche's Harvest Mouse. 



Reithrodontomys dychei, p. 201. 



Part 2. Under parts white or grayish white; upper parts including head rich, 

 golden brown. Southern Golden Mouse. 



Peromyscus nuttalli aureolus, p. 198. 



Part 3. Under parts mouse brown; upper parts darker brown. 



House Mouse. 

 Mus musculus, p. 176. 



SECTION 2. Tail 4 to 5 inches long. 



Hair on tail very scanty; feet decidedly larger and broader than 

 Peromyscus; upper parts brown; under parts grayish. The line of de- 

 marcation between color of sides and belly not decidedly abrupt as in 

 Peromyscus. Rice Field Mouse. Rice Rat. 



Oryzomys palustris, p. 202. 



B. Species which occur in northern Illinois or Wisconsin. 



SECTION 1. Tail more than 2, but less than 3.75 inches long. 



Part 1. Under parts white or grayish white; upper parts brown; middle of 

 back darker brown than the sides. 

 Tail less than 2.75 in. (70 mm. long, usually from 55 to 68 mm.); total 

 length 5.50 to 6.25 in. (140 to 160 mm.). Occurs in Illinois and southern 

 Wisconsin. 



Prairie White-footed Mouse. 

 Peromyscus maniculatus bairdi, p. 190. 

 Tail more than 2.75 in. (70 mm.) long; total length 6.38 to 7.37 in. (162 to 

 188 mm.); tail usually 2.87 to 3.25 in. long (72 to 83 mm.); no trace of 

 whitish hairs at anterior base of ears.* Large specimens are often 

 quite similar to small examples of P. m. gracilis but gracilis does not 

 occur in southern Wisconsin or Illinois. 



Northern White-footed Mouse. 

 Peromyscus leucopus noveboracensis, p. 185. 



* Mr. Wilfred H. Osgood gives the following characters by which P. I. nove- 

 boracensis and P. m. gracilis may be distinguished (Revision of the Mice of the 

 American Genus Peromyscus, N. Amer. Fauna, No. 28, 1909, pp. 35 and 42.): 



"P. m. gracilis. Tail longer, more distinctly pencillate and more sharply bicolor; 



pelage slightly softer, with or without white hairs at the anterior base of the ear. 

 P. I. noveboracensis. Tail shorter and less sharply bicolor, slightly or scarcely 



pencillate; never with white spot at anterior base of the ear." 



