CHAPTER IX. 

 OBJECTIONS AND DIFFICULTIES AND OTHER THEOEIES. 



Two classes of criticism have developed : 



I. That the tritubercular type is not primitive. 



II. That the Cope-Osborn theory of the origin of the superior 

 molars is incorrect. 



Let us consider these in order. 



I. That the Tritubercular Type is not Primitive. 



1. THE TLEXODOXT' OK PROGRESSIVE SIMPLIFICATION THEORY OF 



AMEGHINO. 



In 1884, 1 1896,- and 1899 3 Dr. Florentine Ameghino fully stated 

 an original theory of the origin of the grinding teeth, in which there 

 are four main propositions : 



Firxt (op. cit., 1889, p. 556), that aside from the haplodont 

 reptilian type, the most primitive type of inferior molar is not 

 the protodont, triconodont, or tritubercular, but the pli'.roiJout. The 

 original plexodont upper or lower molar is supposed to have been 

 quadrangular, quadrituberculate, quadriradiculate (op. cit., 1896, pp. 

 18, 19, 25, 61, 64). The oldest known form of complete plexodont 

 molar, that of Protn><H<t<'li>li>/x (Fig. 202 and op. cit., 90, p. 556) 

 has an anterior and a posterior lobe, each of them carrying three 

 cusps, which are designated in the following terms, the names in 

 parenthesis being those of Osborn's nomenclature (op. cit., p. 557). 



ma, median-anterior (paraconid). pe, postero-external (hypoconid). 



ae, autero-external (protoconid). />/, postero-internal (entoconidj. 



ai, antero-internal (metaconid). ///_/>, median -posterior (hypoconulid). 



1 Filof/enia, Svo, 1884. 



2 "Sur 1'Evolution des Dents des Mammiferes," Hal. Aca</. X"<-. ,1, Cienc., T. XIV.. 

 1896, pp. 381-517. 



a "0n the 1'riniitivc Type of the Plexodont Molars of Mammals," /Vor. Zoo/. S<,r. 

 Lond., May 2, 1899. 



