VII.] 



THE EXTERNAL SKELETON. 



259 



roots just like those of the Squirrel or Rat. Such teeth, 

 specially adapted for gnawing, as we so often know to our 

 cost as regards the Rat, are denser on the front surface than 

 elsewhere, so that the action of gnawing, as it wears away 

 more quickly the softer part behind, keeps a constantly sharp 

 chisel-shaped edge in front. Animals which have teeth thus 

 formed are apt to suffer fatally from the loss of one, as, there 

 being then nothing to wear away its natural opponent of the 

 opposite iaw, the latter continues to grow and complete that 



FIG. 226. SIDE VIEW OF THE SKULL OF A PORCUPINE (Hystrix crisiata). 



(TYPICAL RODENT DENTITION.) 



a, angle of mandible ; c, occipital condyle ; cr, coronoid process of mandible ; cy, 

 condyle of mandible ; /, frontal ; z, incisor teeth ; j, ascending branch of 

 maxilla enclosing the enormous infra-orbital foramen, the course of the 

 masseter muscle through which is indicated by an arrow ; in, molar teeth ; 

 tnx, maxilla ; n, nasal ; pm, premolar teeth ; pm, paramastoid process ; px, 

 premaxilla; ^, tympanic bone ; z, zygomatic arch the part formed by the malar. 



circle of which its axis (from root to apex) describes a seg- 

 ment. Rabbits and Hares are not unfrequently found dead 

 from such accidental deformity. 

 A pair of strong but pointed upper incisors are found in 



FIG. 227. DENTITION o? 



DESMODUS. 

 i, incisors ; c, canines. 



FIG. 228. A LOWER INCIOR OF 

 GALEOPITHFCUS, showing its 

 comb-like form. 



the blood-sucking Bat Desmodus, and to these pointed in- 

 cisors notched incisors are opposed below. In another Bat 

 s 2 



