S88 MAMMALS. 



Order 5. Rodentia. 



Rodents are represented in all parts of the world, and by 

 more species than any other order of mammals. Most of 

 them are small and most are terrestrial, but there are some 

 arboreal and aquatic forms. All are herbivorous and gnaw 

 their food in a characteristic way. 



The dentition is characteristic. The incisors have chisel- 

 like edges, for as the enamel is usually entirely restricted 

 to the front of the teeth, the posterior part wears away more 

 rapidly. The incisors are always rootless, growing from 

 persistent pulps as they are worn away, and the same is 

 sometimes true also of the back teeth. On the lower 

 jaw there is never more than a pair of incisors, and in 

 most cases the upper jaw also has only a pair. There are 

 no canines, and the skin projects as a hairy pad into the 

 mouth through the large gap between incisors and premolars. 

 The premolars are always below the typical number. 



The feet are plantigrade or semi-plantigrade, generally 

 with five clawed or slightly hoofed digits. Clavicles, though 

 often rudimentary, are generally present. The scapula has 

 usually a long acromion process. 



The condyle of the mandible is elongated from before 

 backwards, and in gnawing the jaw moves backward and 

 forward (unimpeded by any postglenoid process of the 

 squamosal). The mandible has an abruptly narrowed and 

 rounded symphysis, and a very large angular portion. The 

 orbits are confluent with the temporal fossse. The zygomatic 

 arch is complete. There is generally a distinct interparietal 

 bone. The tympanic bullae are always developed, and are 

 often large. 



The cerebral hemispheres are smooth and leave the 

 cerebellum uncovered. 



The skin is generally thin, and the panniculus carnosus 

 but slightly developed. 



The intestine has a large caecum, except in Myoxidse. 

 Special anal or perineal or other glands secreting odoriferous 

 substances are usually developed. 



The testes are inguinal or abdominal in position; only in the 

 hares and rabbits do they completely descend into scrotal sacs. 



