866 PHYLUM CHORDATA : CLASS MAMMALIA 



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

 away more rapidly. The incisors are rootless, growing 

 from persistent pulps, and the same is sometimes true of 

 the bunodont or lophodont back teeth. There is never 

 more than a pair of lower incisors, and in most cases the 

 upper jaw has only a pair. There are no canines, and the 

 skin projects as a hairy pad into the mouth through the gap 

 between incisors and premolars. 



The feet are plantigrade or semi-plantigrade, generally 

 with five clawed 3r slightly hoofed digits. Clavicles, 

 though often rudimentary, are generally present. The 

 scapula has usually a long acromion process, sometimes 

 with a metacromion. 



con 



Fig. 514. — Lower jaw^of a rodent. — From^a Specimen. 



Showing chisel-edged incisor tooth (i.) ; the diastema (d.) or gap, 

 where in most other orders the canines would be ; pm., first 

 premolar ; a., angle of the mandible ; con., condyle of the 

 mandible ; c.p., coronary process. 



The condyle of the mandible (and the corresponding 

 articular surface for it) is usually elongated, and the jaw 

 moves backward and forward. 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, but the 

 jugal is restricted to the middle of it. The premaxillae are 

 large, the palatines small. There is generally a distinct 

 interparietal bone. The tympanic bullae are always de- 

 veloped, and often large. 



The cerebral hemispheres are almost without convolu- 

 tions, and leave the cerebellum uncovered. The skin is 

 generally thin, and the panniculus carnosus but slightly 



