(4he(;()ry: aotharctus, an American eocene primate 



133 



Third Lower Premolar 

 Plates XXXVI, XXXVII 



The most ancient and piiniitive p:* known in the Notharctinas is that of Pelijcodus ralstoni from the 

 Sand Coulee horizon at the base of the Lower Eocene (Amer. Mus. No. 16093). It has a compressed, 

 slightly recurved crown with sharp anterior and posterior cristse and distinct internal cingulum, the 

 latter rising on the middle of the innej' surface of the protoconid into a slight projection. The stout 



Fig. 38. Occlusal rolations oi the upper ancl lower cheek teetli of yofluirctits eriissus at tlie inonient of maximum lat- 

 eral displacement of the mandible. Amer. Mus. X'o. 119S2. 



The protoconids (pr'') fit between two upper molars, the hypoconids (/(;/') fit between the para- {pa) and metacones {me); the proto- 

 conid-paraconid crest shears past the metacone-metastyle crest. The metaconid articulates between the pseudohypocone {ps-hy) 



and the protoconule. The entoconid (en'') articulates between the protocone and the pseudohypocone. As the mandible is closed 

 it passes linguad, bringing the protocones into the central fossae, or basins, of the talonids and the pseudohypocones into contact with the 

 paraconid region. 



posterior cingulum rises in the middle into a low cusp which is j^robably homologous with the entoconid 

 of the molars and is continuous with the crista posterior; lateral to the crista posterior and supposed 

 entoconid is a slight depression which seems to represent the central external valley of the molars ; medial 

 (lingual) to the above named crest is another and slightly deeper fossa which appears to represent the 

 talonid basin. The talonid as a whole, though small, is well marked; it is supported by a root which 

 on the inner side is much larger than the one beneath the potential trigonid. 



In Pelycodus trigonodus (Amer. Mus. Nos. 16843, 15017) ps is somewhat more compressed and elon- 

 gate and the internal cingulum is somewhat less sharply defined. In P. jarrovii, which is one of the larger 

 species of the genus, the crown of ps becomes thicker transversely ; the crista posterior gives rise to a medial 

 branch ending below in a slight swelling or ridge, which appears to be prophetic of the metaconid. In 

 Nothardus venticolus the tip of the tooth is lower, the supposed metaconid ridge is barely perceptible; 

 in N. tyrannus (paratype, Amer. Mus. No. 11478) the metaconid swelling is clearly indicated and the 

 whole crown is thicker and lower; this tendency culminates in N. pugnax and A^. crassus, which have 

 the crown of ps cjuite swollen transversely, with the metaconid swelling now conspicuous. This extremely 

 gradual evolution of the metaconid on ps is analogous with the similarly slow appearance of the meso- 

 styles and pseudohypocones of the upper molars. 



In N. venticolus (Amer. Mus. No. 14655) the posterior flattened slope of the potential trigonid of ps 



