SKCON'I) ori'LINK OF TKlTrilKIK'l'LY 81 



the third stage, retaining- the term " tritubercular " as descriptive of 

 the whole transformation, and as peculiarly appropriate to certain types 

 of teeth, such as the superior molars of the lemurs. " Trigonodont ' 

 is most appropriate because the first step in molar morphology is to 

 identify the "primitive triangle," and the term "tubercular" hardly 

 applies to a lofty pointed cutting crown. Our studies among the Mesozoic 

 mammals have left no doubt that the upper and lower triangles, or 

 " trigon " and " trigonid," were derived from the reptilian protocone by 

 the addition of lateral cusps. The mechanical perfection of this type 

 consisted in the fact that the lateral cusps were developed upon or 

 shifted to the outer side in the upper molars, and to the inner side 

 in the lower molars, thus producing an interlocking " shear." The 

 " trigon " was essentially a cutting apparatus, so perfect that many 

 mammals retained it without further evolution. Thus Chrysoddoris, the 

 little Insectivore of the Cape, presents a fine example of this type, 

 persistent in its molars* (Fig 42*). 



The Talon. But in a great majority of tritnberculates the " talon " 

 was added as a crushing apparatus. It invariably appeared first in the 

 lower molars (where we may distinguish it as the " talonid ") and pressed 

 into the basin of the superior " trigon." At first it was a mere spur 

 (hypocone) as in Amphitherium or in the existing Calcochloris (allied to 

 Chrysochloris), but between the Jurassic and Upper Cretaceous periods the 

 talonid widened into a basin-like shelf supporting an outer cusp, the 

 ' hypoconid " ; an intermediate cusp, the " hypoconulid," and an inner 

 cusp, the " entoconid." Thus we find in the majority of the Upper 

 Cretaceous (Laramie) and Puerco or lowest Eocene mammals that the 

 lower molars bear six cusps ; the above-mentioned three on the talonid 

 and three on the trigonid (protoconid, paraconid, metaconid). With these 

 six cusps the equipment of the lower molar was complete, and it was 

 ready for transformation into the molar of a primate, ungulate or 

 carnivore, as the case might be. 



But why notice such a detail as the posterior intermediate cusp or 

 hypoconulid ? Because, to give only two reasons, this cusp plays an 

 important role in the ungulates; it is invariably present, t except perhaps 

 in the Coryphodons, and forms the third lobe of the last lower molar, 

 which is thus proved to be a primitive character ; again, it is found 

 throughout all the Primates, and although seldom availed of, this cusp 

 constitutes an important and distinctive character as between the different 

 races of man. Its extreme antiquity is appreciated by few anthropologists, 

 and at the present time it is degenerating. (See Fig. 40, Nos. 11, 12.) 



* [It should be borne in mind, however, that some authors (e.'/. Fovsyth Major) hold 

 that the heelless condition of the Chn/ii,,-/,/,,, <x molars is secondary. See also pp. 124, 

 _'_>.-> below. ED.] 



t[That is, in the more generalized forms. Kn.] 



F 



