230 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



•2 1 23 .51 3 4 



1 > c I pm t m ; that of the opossum by 1 > c > pm 1 m • 

 21^23' ^ ■'4i'^34 



Not infrequently the enamel is lacking from the teeth of mammals, as in 

 whales, dugongs and edentates, or it may be restricted to one side of a tooth, as in 



A4^ 



? 



AA 



n » 



AA,<Js. 



iw 



p 



Fig. 242. — Diagram of the relation of the human teeth to the primitive dentition, 



after Rose. 



the/ incisors of rodents. Sexual differentiations occasionally occur in mammals, 

 certain teeth (usually canines or incisors, more rarely premolars) being better 

 developed in the males than in the females of the same species. 



Fig. 243. — Epidermal tooth (black) of lamprey, after Warren, c, corium; e, 

 epidermis; m, Malpighian layer; p, papilla of corium in epidermis; r, replacement tooth 

 developing. 



There are two views as to the way in which the complicated molars of the 

 mammals have arisen. Both start with the conical tooth as the primitive condi- 

 tion. One theory is that the fusion of such simple teeth is sufl&cient to account 

 for the multiplication of roots and tubercles in all of their varying forms (figs. 



