THE TEETH 59 



the teeth. Those of the rabbit present two characteristically 

 mammalian features; they are heterodont, or differentiated in 

 different regions of the jaw; and they are diphyodont, that is to 

 say, the adult series are permanent teeth, which, excepting those 

 designated as molars, replace deciduous, or milk teeth of the 

 young animal. In lower vertebrates, in contrast with this con- 

 dition, there is usually a multiple tooth change, new teeth being 

 developed as required (polyphyodont type). Moreover, in the 

 rabbit, as in all mammalia, the number is restricted, so that, con- 

 sidering the differentiation of the teeth, it is possible to express 

 their relations by a dental formula. In the mammalia generally 

 the teeth are differentiated into incisors, canines, premolars, and 

 molars, and in placental mammals the full dental formula is indi- 

 cated as i. I, c, T, pm. |, m. f . The incisors, the most anterior 

 teeth, are more or less flattened into a chisel-like form; the canines 

 are sharp, roughly conical, and sometimes elongated into prom- 

 inent tusks; the premolars and molars, together designated cheek- 

 teeth, are distinguished chiefly by the occurrence of the former 

 in both sets and of the latter in the permanent set only. In the 

 rabbit, as in other rodents, however, the dentition is greatly modified 

 by the elaboration of two pairs of incisors for gnawing and the 

 corresponding obliteration of intermediate teeth, the place of the 

 latter being occupied by an extensive gap, or diastema, in which no 

 teeth occur. The dental formula of the rabbit is i. f» c. J, pm. f , 

 m. f. The specialization of the medial incisors retained by the 

 rabbit is accomplished by the elimination of enamel from the 

 posterior surfaces so that the posterior part of the tooth is worn 

 away by use more rapidly than the anterior layer, which thus 

 forms a sharp cutting edge. It will also be observed in this animal 

 that the absence of the intermediate teeth allows the lips to be 

 approximated behind the incisors, and since in this region the lips 

 are also provided with hairs on their internal surfaces, the main 

 part of the oral cavity is separated almost completely from a 

 small space enclosing the incisor t^eth. This adaptation, however, 

 is not so perfectly developed in the rabbit as in certain members 

 of the rodent order proper. 



The cheek-teeth of the rabbit are modified for grinding by the 

 formation of flattened ends with prominent transverse ridges. 



