190 Bulletin American Museum, of Natural History. [Vol. XXVII, 



Eocene Oxy eleven idte. The trigonid itself had formerly merely filled the 

 interspace between successive protocones but its basin now began to be 

 occupied by the postero-internal cingulum, or hypocone. The subsequent 

 disappearance of the always low paraconid, which interfered with the 

 development of the hypocone has also been noted by Osborn. But a cardinal 

 fact which has not hitherto been sufficiently emphasized is that tlie hypocone 

 is an accessor;/ crusher, fitting into the basin of the trigonid of the succeeding 

 lower molar, just as the protocone fits into the talonid of the corresponding 

 lower molar (cf. Gregory, in Osborn, 1907, p. 61, foot-note; cf. Figs. 12, 13 

 below). Inasmuch as the protocone and hypocone had a very different 

 origin, the subsecjuent similarity of the two cusps furnishes an excellent case 

 of convergent evolution among cusps, and this case helps us to understand 

 how premolars and molars may sometimes attain a perfect similarity of 

 pattern and yet consist partly of non-homologous components. 



In the later mammals the protocone became depressed as the talonid 

 grew up, while the trigonid became depressed as its pestle, the hypocone, 

 developed; so that finally the anterior and posterior moieties of the lower 

 tooth attained practically the same level. 



Broadening of the talonid and origin of the hypoconid. The talonid was 

 at first merely a narrow posterior spur, rising internally into the entoconid. 

 This condition is retained in the Jurassic Trituberculates and in the Eocene 

 and modern Zalambdodont Insectivores. ■ The protocone at first fitted into 

 the space external to the talonid. As the talonid widened transversely a 

 small cusp, the hypoconid, grew up, fitting l)ehind and external to the pro- 

 tocone, further widening of the talonid caused the protocone to be received 

 into the basin of the talonid internal to the hypoconid, and at the same time 

 the hypoconid was received into the central valley of the trigon above (Fig. 

 12, 13). 



In order to summarize the foregoing observations on the spatial relations 

 of the parts of the upper and lower molars in a generalized tritubercular 

 stage, the following table is inserted: 



Functions and spatial relations of the parts of the upper and lower molars 

 in a generalized tritubercular dentition {cf. Figs. 11, 12, 13). 



I. Overhanging cusps (serving either as points of insertion or as fulcra). 

 Upper molars: Parastyle, external cingulum, metastyle, external face of 



paracone and metacone. 

 Lower molars: Paraconid, metaconid (at first filling space between 



successive protocones). 

 Entoconid, internal to protocone of corresponding 

 upper molar. 



