Orpver 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 100 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 Sciurid@, but is imperfect or rudimentary in the Leporide. 
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, 
those which have 4 incisors in the upper 
jaw, the second pair being very small and 
placed directly behind the others. The 
latter suborder contains but two living 
families, the Ochotonide and the Leporide 
in which are included our Rabbits and 
Hares. No living Rodent has more than 
two lower incisors, and those belonging to the family Muride have 
but three cheek teeth (molars) and apparently lack milk dentition. 
Other important characters for the order are a large cecum, which is 
nearly always present, testes abdominal or inguinal and placenta 
discoidal and deciduate. All the members of the order are more or 
97 
Skull of a Rodent. 
