Order GLIRES. 

 gnawing mammals. 



The order Glires, formerly known as Rodentia, has the distinction 

 of being the largest as well as the most widely distributed throughout 

 the world. Its members are characterized by strongly developed 

 chisel-like incisors in both jaws and the absence of canine teeth, 

 leaving a wide gap between the large chisel-shaped front teeth and 

 the "back teeth." There is a great diversity in form and habits 

 among its members: some are strictly terrestrial; others, such as the 

 Muskrat, are semi-aquatic, passing the greater part of their lives in the 

 water; others again are arboreal, such as many of the Squirrels. In 

 size they vary from the diminutive Mouse to the South American 

 Capabara (a relative of our familiar Guinea Pig), the giant of its 

 order, often weighing more than loo pounds. 



There is also considerable variation in the osseus systems of the 

 various families, as, for example, the clavicle (collar bone) is well devel- 

 oped in the Sciuridce, but is imperfect or rudimentary in the Leporidce. 

 A zygomatic arch is always present but the position of the jugal 

 therein is variable. In the Rats and Mice the tibia and fibula (lower 

 leg bones) are fused together, but are separate in the Squirrels and 

 Porcupines. However, such variations would be expected in animals 

 of such great diversity of habits, but whatever other skeletal differ- 

 ences exist the dental characters are peculiar and diagnostic. 



The members of the order are separated into two suborders: I, 

 Simplicidentata or simple toothed. Rodents having but 2 incisors in 



the upper jaw; and II, Duplicidentata, 



^^w^Ir^'T "^W\?^?> those which have 4 incisors in the upper 



J^^Rsi^^^^^^^ibM J^^' ^^^ second pair being very small and 



<J^^^^M^mif^ rJ placed directly behind the others. The 



•^f \^^^,^-~j^y latter suborder contains but two living 



^"^^^^ftis^^ families, the OchotonidcB and the Leporidce 



in which are included our Rabbits and 



Skull of a Rodent. tvt i- • -r> i i i 



Hares. No living Rodent has more than 

 two lower incisors, and those belonging to the family Muridce have 

 but three cheek teeth (molars) and apparently lack milk dentition. 

 Other important characters for the order are a large cascum, which is 

 nearly always present, testes abdominal or inguinal and placenta 

 discoidal and deciduate. All the members of the order are more or 



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