Rodentia 
29 
usually with a large caecum; testes inguinal or abdominal; 
uterus two-horned; placenta discoidal and deciduate. 
The rodents are mostly small animals of herbivorous 
habits adapted to terrestrial^ arboreal, subterranean, or 
occasionally to natatorial life; the order contains a much 
greater number of species than any other; rodents are cos- 
mopolitan, being found all over the world, though perhaps 
more abundantly represented in South America at the present 
day than elsewhere. 
The following key is only put together to distinguish 
Colorado families and genera. 
Key of the Colorado Suborders and Families 
A. With four incisors in the upper jaw, the second pair small and 
placed directly behind the front pair. (Duplicidentata.) 
a. A short bushy tail; ears long and narrow. Leporidae. 
b. No external tail; ears short and rounded. Ochotonidae. 
B. With only two incisors in the upper jaw. (Simplicidentata.) 
a. Body partially at any rate covered with stout, stiff, sharp 
spines which are modified hairs. Erethizontidse. 
b. Hairs covering the body never modified into stout, stiff spines, 
a'. Hind limbs more or less kangaroo-like, and adapted for 
jumping. 
a". Without external, but with small internal cheek pouches ; 
toes five in number, metatarsals separate. Zapodidae. 
b". With external cheek pouches; toes five in number. 
Heteromyidae. 
b'. Fore and hind limbs subequal; the hind limbs never 
kangaroo-like. 
a". With well developed external cheek pouches opening 
on either side of the head. Limbs stout and 
adapted for digging. Geomyidae. 
W. No external cheek pouches. 
a Tail round and scantily haired, or naked and scaly; 
skull without postorbital processes. Muridae. 
b Tail broad, flattened horizontally, naked and scaly; 
skull with postorbital processes. Castoridae. 
c "\ Tail round, more or less hairy or bushy, without 
scales; skull with distinct postorbital processes. 
Sciuridae. 
