CHAPTEE 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 ^, 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 imperfect 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 fossae ; the condyle of the 

 mandible is elongated 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 Myoxidce) has a large caecum ; 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 include 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- 



