CHAPTER X 
THE ORDER RODENTIA 
THE Rodentia, or Rodents, form a well-defined order, readily dis- 
tinguished by their large scalpriform incisors and the absence of any 
trace of canines. The existing forms are mostly of comparatively 
small size, and are generally of terrestrial habits, although a 
few are arboreal or natatorial. The dentition is diphyodont ; the 
mandible never has more than a single pair of incisors; the pre- 
molars are always below the full number, being very generally 4, or 
altogether wanting. The feet are plantigrade or semi-plantigrade, 
generally with five digits, and usually unguiculate, although occa- 
sionally of a subungulate type. Clavicles are present as a rule, 
although they may be imperfeet or rudimentary. 
The upper incisors resemble the lower in growing uninter- 
ruptedly from persistent pulps, and, except in the suborder 
Duplicidentata, agree with them in number; the premolars and 
molars may be rooted or rootless, with tuberculated or laminated 
crowns, and are arranged in an unbroken series. The orbits com- 
municate freely with the temporal fosse; the condyle of the 
mandible is clongated in the antero-posterior direction, and, through 
the absence of a postglenoid process to the squamosal, admits of a 
backward and forward motion of the jaw. The intestine (except 
in the J/yoride) has a large excum; the testes are inguinal or 
abdominal; the uterus is two-horned, the cornua either opening 
separately into the vagina or uniting to form a corpus uteri; the 
placenta is discoidal and deciduate ; and the smooth cerebral hemi- 
spheres do not extend backwards so as to cover any part of the 
cerebellum, 
The Rodents inelude by far the greatest number of species, and 
have the widest distribution of any of the orders of terrestrial 
mammals, being in fact cosmopolitan, although more abundant in 
some parts than in others. The total number of known existing 
species exceeds 900. South America may be regarded as their head- 
