7io MAMMALIA. 



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 either entirely restricted to 

 the front, or is at most thin posteriorly, the back part wears 

 away more rapidly. The incisors are always rootless, grow- 

 ing 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 pre- 

 molars. 



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 fossae. 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 cere- 

 bellum uncovered. 



The skin is generally thin, and the panniculus carnosus 

 but slightly developed. 



The intestine has a large caecum, except in Myoxidae. 

 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. 



