DENTAL SYSTEM OF MAMMALIA. 



269 



210 



assume places not previously occupied by deciduous ones, may be 

 regarded as a continuation of that series, and are posterior in their 

 position ; they are generally the most complex in their form. The 

 successors of the deciduous incisors and canines differ from them 

 chiefly in size. The successors of the deciduous molars may differ 

 likewise in shape, in which case they have less complex crowns 

 than their predecessors. The i bicuspids ' in Anthropotomy, fig. 

 258, p 3, p 4, and the corresponding teeth called ' premolars ' in 

 lower mammals, fig. 293, p 2-4, illustrate this law. 



The Mammalian class might be divided, in regard to the succes- 

 sion of the teeth, into two groups — the Monophyodonts, or those 

 that generate, as a rule, one set 

 of teeth, and the Diphyodonts, 

 or those that generate two sets 

 of teeth. 1 The Monophyodonts 

 include the Monotremata, Ceta- 

 cea and Bruta; all the other 

 orders are Diphyodonts. 



The teeth of Mammalia, espe- 

 cially of the Diphyodonts, have 

 usually so much more definite 

 and complex a form than those 

 of fishes and reptiles, that three 

 parts are recognised in them : the 

 fang or root (radix, fig. 210,/) 

 is the inserted part ; the crown 

 {corona, ib. k) is the exposed 

 part ; and the constriction which 

 divides these is called the neck 

 (cervix, ib. n). The term 'fang' 

 is properly given only to the 

 implanted part of a tooth of re- 

 stricted growth, which fang gradually tapers to its extremity. 

 Those teeth which grow uninterruptedly, fig. 236, have not their 

 exposed part separated by a neck from their implanted part, and 

 this generally maintains to its extremity the same shape and size 

 as the crown. 



It is peculiar to the class Mammalia to have teeth implanted in 

 sockets by two or more fangs, figs. 256, 293 ; but this can only 

 happen to teeth of limited growth, and generally characterises the 

 molars and premolars : perpetually growing teeth require the base 

 to be kept simple and widely excavated for the persistent pulp, 

 and 216. In no mammiferous animal does anchylosis 

 1 Vol. ii. p. 268. 



Section of human molar tooth, magn. 



figs. 215 



