TRITUBERCULAR EVOLUTION IN MAMMALS 41 



lower cusps. The first "secondary' cusps (hypocone, hypoconid), 

 which are added to the upper and lower molars of the primitive 

 triangle, modify the crown from a triangular to a quadrangular shape. 

 and hence may be considered homologous. The three additional 

 secondary cusps (protoconule, metaconule, entoconid) evidently have 

 no homology with each other. 



TERMS NOW IN USK PROPOSED TERMS. ] ABHKKV. 



Upper Molars. 



Antero-internal-cusp, ....... Protoconc. pr. 



Postero- or 6th cusp Hypoconc. hy. 



Antero-external ........ Paracone. pa. 



Postero- ,, ,,....... Metacone. me. 



Anterior Intermediate cusp, Protoconule. pi. 



Posterior ,, Metaconule. ml. 



Lower Molars. 



Antero-external cusp, ....... Protoconid. pr d . 



Postero- ,, Hypoconid. hy d . 



Antero-internal cusp, or 5th cusp, .... Paraconid. pa d . 



Intermediate or antero-internal cusp (in quadrituber- 



cular molars), ....... Metaconid. me d . 



Postero-internal cusp, ....... Entoconid. en d . 



Evolution of the Cusps. The cusp evolution in the Mesozoic period 

 has been fully discussed by the writer (op. cit., pp. 240-4) and in 

 the Tertiary period, by Professor Cope, so that only a brief resume 

 is necessary here. In Dromatherium (Fig. 3), from the upper Triassic, 

 the oldest mammalian type known, with the exception of Microlcstes, 

 the molars have a main protoconicl with several minute lateral cuspules, 

 differing in size in the different teeth, but in general giving a trifid 

 appearance to the crown. The molars of the contemporary Microconodon 

 (Fig. 3) also have impaired fangs, but distinctly trifid crowns, with 

 the anterior and the posterior cusps, or para- and meta-conids, upon 

 the slopes of the protoconid. This Triconodont type reappears, with 

 the addition of a cinguluni and paired fangs, in Am i>Ii listen (Fig. 5) 

 and Phascolotlicrium (Fig. 6) of the lower Jurassic and persists in 

 Triconodon (Figs. 7, 8) of the upper Jurassic. In this succession we 

 observe especially the relative subsidence of the protoconid and upgrowth 

 of the para- and meta-conids. Contemporary with Amphilcstcs is the 

 classical genus Amphitherium (Fig. 15). A recent examination of 

 the type specimen by the writer revealed the very interesting fact 

 that the molars of this genus are probably of the primitive tubercular- 

 sectorial type the oldest known example. Only the paraconid, 

 metaconid and hypoconid have been observed heretofore, but one can 

 see the tip of the main external cusp between the internal pair. 



^am much indebted to my colleagues Professors Macloskie and Winaus for assiM 

 ance in the selection of these terms. 



