92 The Mammals of Colorado 
dentition (in all Colorado forms): i. {; c. pm. |; 
m. I X 2 = i6; lower incisors compressed; no premolars; 
molars rooted or rootless, tuberculate or with angular enamel 
folds. 
This is a cosmopolitan family with a very large number of 
genera and species, including more than a third of all existing 
rodents. 
Key of the Subfamilies and Genera 
A. Molars rootless or imperfectly rooted, composed of longitudinal 
rows of triangular prisms. (Microtinae.) 
a. Animals of small size, under lo ins. total length; tail short 
and less than half the length of the body, and rounded, 
a'. Angles on the outer and inner sides of the lower molars 
approximately equal, 
a". Color of back never rufescent; molars quite rootless, 
Microtus, p. 93. 
b". Color of back always rufescent; molars quite rootless. 
Evotomys, p. 10 1, 
b'. Molars with two roots in adults; angles on the outer side 
of the lower molars far deeper than those on the inner. 
Phenacomys, p. 103. 
b. Large animals, total length about 20 ins.; tail laterally 
compressed and flattened and fringed with stiff hairs. 
Fiber, p. 105. 
B. Molars rooted but not tuberculate, with folded enamel plates 
resembling those of MicrotincB. (Neotominae.) 
Animals of large size over 10 ins. total length with long tails, 
sometimes bushy, sometimes only thickly haired. 
Neotoma, p. 108. 
C. Upper molars with tubercles arranged in two longitudinal 
rows. (Cricetinae.) 
a. Incisors grooved; tail more than half the length of the body. 
Reithrodontomys, p. 121 
b. Incisors smooth, without grooves. 
a'. Tail less than 3^ length of body; sole hairy on posterior 
half, with only four small anterior pads. 
Onychomys, p. 124 
b'. Tail more than ^ length of body; sole naked, with the 
usual six pads. Peromyscus, p. 127 
D. Upper molars with tubercles arranged in three longitudinal 
rows. (Murinae.) 
