DIGESTIVE ORGANS. 



215 



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 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. 



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

 mals have arisen. Both start with the conical tooth as the primitive condition. 

 One theory is that the fusion of such simple teeth is sufficient to account for the 

 multiplication of roots and tubercles in all of their varying forms (figs. 217, 218). 



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FIG. 217. Diagram of *he relation of the human teeth to the primitive dentition, after 



Rose. 



The other hypothesis is that parts have been developed on the primitive cone, 

 giving, first, the triconodont shape. Next these three cones have been shifted to 

 the tritubercular position; and later other parts hypocone, lophs, etc. have 

 been added and these have been modified in different directions. Each view has 

 much in its favor. Embryology is not at all decisive, while paleontology favors the 

 latter view. 



Epidermal Teeth occur in cyclostomes and in larval amphibia and 

 in embryonic monotremes. In the cyclostomes they are cones of 

 cornified epithelium covering an underlying core of the integument; they 

 are differently arranged in the lampreys and myxinoids. In the latter 

 they are few, there being a single tooth on the 'palate' and two chev- 

 ron-shaped rows on the tongue. In the lampreys nearly the whole 

 inner surface of the oral hood is lined with these teeth of varying shape, 



