Order VI. Rodentia. Rodents. 
Coues and Allen, Monographs of North American Rodentia, U. S. 
Geol. Survey, 1877. 
The Rodents constitute the largest Order of Mammals, and the 
numerous members possess a great diversity of form. They are 
readily distinguished among all mammals by their incisors, four in 
number, two above and two below, (except Hares and Rabbits, which 
have a supplementary upper hinder pair in adults,) curved hollow 
tubes filled with pulp, hardened at the surface, .he portion beneath 
the gum curving and sometimes traversing the length of the jaw bone. 
The species are mostly small, the harvest mouse being the pigmy, 
from which genus the size increases until the comparatively great 
beaver is reached, and he is exceeded in bulk only by the capybara 
of South America. The majority, however, are small animals, and 
their habits are as diverse as their shapes, and we find among them 
not only terrestrial and aquatic creatures, but others which are pro- 
vided with extensible membranes between the limbs and body, to 
enable them to traverse the air as if carried by parachutes. Rodents 
are cosmopolitan, the greatest number being found in South America, 
the fewest in Australia. They are mostly herbivorous, yet some, like 
the ordinary rat, are omnivorous. The incisors have a continuous 
growth and are worn away at the terminal portion by constant gnaw- 
ing or by attrition. The molar teeth are usually rootless, and their 
crowns often present many varied, even intricate, patterns of enamel 
folds and loops. No canine existsin any rodent. Normally the species 
of this family generally have no premolars, although in some a small 
one is present, and among squirrels two on each side above, and one 
below are found, but the additional premolar is frequently deciduous. 
The diversity of form and habits is very great in the members of this 
order, and we have the tree-loving, graceful squirrel in countless 
colors, and its small imitator, the chipmunk; ground squirrels that 
live in burrows, and flying squirrels darting through the air; the 
innumerable field mice of many genera and species; rice and cotton 
rats, pouched rats that live under ground and tunnel long galleries 
like the moles; jumping mice with long hind legs and greatly length- 
ened tails that possibly may assist their owners in making the 
kangaroo-like leaps over the fields; aquatic rats whose home is in the 
water, and whose feet are formed more for swimming than walking— 
all these, and more, help to constitute the great order of the Gnawers. 
89 
