MAMMALS. 389 



a chisel-like edge ; J the persisting pulp renews all loss by wear. 

 Between the incisors and the molars is a wide gap or diastema. 

 The molars may be lophodont, bunodont, or prismatic. As a 

 whole, the dentition varies between i f , c , / $, m (hares) 

 and i \, c %, p %, m f ; the most usual being z \, c $, p , w $. 

 The milk dentition does not include the incisors as a rule, and 

 in some cases, as the guinea-pigs, is lost before birth. 



The skin may be covered with the softest fur (chinchillas), 

 or certain hairs may be developed into enormous spines, as in 

 the porcupines ; or again, the spines may be flattened ; not 

 infrequently are there scales on the tail. Sternebrae occur in 

 the sternum. The skull usually presents an interparietal bone ; 

 the nasal bones are large and long ; the orbits and the temporal 

 fossae are confluent, and especially characteristic is an infra- 

 orbital canal through the zygomatic . process of the maxilla. 

 The clavicle may be present or absent ; the manus is usually 

 pentadactyl, but the thumb may be reduced, while in the hind 

 foot both hallux and minimus may be lost. 



Usually (except myoxidae) there is a large intestinal caecum ; 

 the brain is small, and the cerebral hemispheres, which never 

 cover the cerebellum, are smooth. The testes are inguinal or 

 abdominal in position, while there is either a uterus bicornis or 

 two distinct uteri. The mammae vary extremely between the 

 two found in guinea-pigs and the ten in some rats. 



About nine hundred living species of rodents are known, 

 and they occur in all regions of the world except the Australian. 

 They are mostly small and are mostly arboreal ; although terres- 

 trial, burrowing, and aquatic species occur. All are herbivorous. 

 The order appears in the eocene, but has its greatest develop- 

 ment in the present time. The genera are not equivalent to 

 those in the preceding orders. 



SUB-ORDER i. SCIUROMORPHA. 



One incisor in the upper jaw, molars f ; clavicle present ; tibia and fibula 

 distinct. Mostly belong to the northern hemisphere, where they appeared 

 in the eocene. 



1 Similar conditions exist among the diprotodont marsupials, in Tyfotherium and in 

 some multituberculates. 



