-202 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



Second, that the plexodont molar made its appearance suddenly 

 (p. 571, bottom), the only theory which can explain it in a satis- 

 factory manner being that it arose by the fusion or the concrescence 

 of " the dental germs or embryos of several simple teeth " (op. cit., 

 '99, p. 555). 



Third (p. 556, 1), that the tritubercular, triconodont and pro- 

 todont stages are secondary simplifications of the plexodont stage. 



Fourth, that the plexodont type is the starting point of all the 

 post canine cheek teeth including the premolars. (The teeth of Mouo- 

 tremes, Edentates, Cetaceans are supposed to have been independently 

 derived from the haplodont type.) 



The ' plexodont ' type of Ameghino as illustrated in Protcodi- 

 dclphys (Fig. 202) is identical with the six-cusped ' tuberculo-sectorial ' 



FIG. 202. Lower jaw and teeth of Proteodidetphys prcecursor, a small Polyprotodont Marsupial 

 from the Chubut Formation. (? Upper Cretaceous) of Patagonia, xi. The true molars (" 5?," 

 "6m," "7m") are in a very primitive stage of the tuberculo-sectorial modification. After 

 Ameghino. 



type of Cope ; briefly stated, therefore, the plexodont theory is that 

 this type, instead of being an intermediate stage, as in the Cope- 

 Osborn theory, is a primordial stage from which, on the one hand, 

 the tritubercular, triconodont and protodont types have been derived 

 by retrogressive simplification or loss of cusps, and, on the other hand, 

 the higher omnivorous (bunodont) and herbivorous (hypselodont) stages 

 have been derived by progressive modification of the relative size 

 of the different parts without the addition of new cusps (p. 565, 

 op. cit.}. In the 1899 paper Dr. Ameghino demonstrates this unity 

 of type in all the South American mammals, namely the Marsupialia, 

 Toxodontia, Litopterna, Primates, etc. 



We give below several of this author's ingenious arguments (op. cit. 

 1896). 



P. 19. Teeth with a single root (e.g. incisors) and simple crown 

 are primitive. Molars with two, three or four roots generally repre- 

 sent the fusion of two, three or four teeth. But roots may coalesce 

 as well as crowns, so that the number of original teeth may be 

 greater in many cases than that of the roots. Coalesced roots often 



