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, t he portion beneath 

 the gum curving and sometimes traversing the length of the jawbone. 

 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 exists in 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. 



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