GREGORY: NOTHARCTrS, AN AMERICAN EOCENE PRIMATE 



137 



the crista obliqua with the raised posterior cingukim, which is potentially the posterior Umb of the 

 posterior A. 



In Pelycodus jarrovii the crown of p4 shows some degree of advance, for the tooth is now wider trans- 

 versely, the trigonid crown is lower and the cusps have lost their thorn-like character. This transverse 

 thickening and blunting of p4 is emphasized in N. venticolus of the Wind River formation and culminates 

 in N. pugnax and A^". crassus of the Bridger. In these species p4 is very coarse and thick and in the much 

 worn condition shows a considerable advance toward the molar pattern. The evolution of p4 like that 

 of all the teeth is thus traceable through a considerable series of small gradations, extending from the base 

 of the Lower Eocene to the top of the Middle Eocene. (Plates XXXVI, XXXVII.) 



The articulation of p4 with p'' and p^ has been intensively studied chiefly in Nothardus venticolus, N. 

 osborni and N. crassus and less fully in Pelycodus trigonodus. The potential trigonid of p4 was wedged 

 between p'^ and p^; its sharp metaconid did not oppose any part of the upper teeth, but as it pressed into 

 the interval between the protocone of p'* and that of p^ it would pierce and break the food, and, on the 

 transverse movement of the jaw, it would tend to tear the food apart. In a subvertical movement of 

 the mandible the cutting protoconid-metaconid edge of p4 and the steep wall below it, when shearing past 

 the parastyle-paracone blade and associated wall of p^, would cut the food; in a transverse movement 

 of the mandible it would tend to tear it apart. The talonid crest, pressing against the protocone tip, 

 would also assist in cutting and breaking the food. Repeated examination of the interlocking relations 

 of the teeth and of the construction of the glenoid and condyle shows that in the later species there was 

 a very considerable transverse motion of the mandible, great pressure being exerted first on the outer 

 side of the upper and lower teeth, where the principal cutting action took place and where the food was 

 firmly gripped by the main external cusps; as the teeth were pushed further into place the lower tooth 

 row moved inward, over a wider arc in the front part of the tooth row than in the rear, so that the food 

 was finally pressed between the protocones and the talonids of p^-ni^ In the later forms with more 

 transverse excursion of the mandible the wider fourth premolars were more effective in cutting, crushing 

 and tearing apart the food; while in the small earlier types a more direct chopping motion was sufficient 

 to cut and break up the brittle insect food. (Plate XLI; Fig. 38.) 



In Adapis parisiensis p4 is almost molariform. The anterior and posterior V's are complete, although 

 the anterior one is still widely open. Even a distinct entoconid was present and the prominent metaconid 

 ridge was nearly of the molar type. In Adapis viagnus p4 is even more molariform. On the posterolingual 

 slope of the metaconid ridge it bears a small accessory cusp (metacristid) homologous with that of the 

 molars. The summit of the protoconid-metaconid crest is sharp and obliquely placed; it sheared past 

 the sharp oblique protoloph of p^ This sharp oblique development of the protoloph of p^ and of the 

 protoconid metaconid ridge of p4 is characteristic of the Adapinae and is associated with a lesser transverse 

 excursion of the mandible, and with a flattened convex condyle. It is evident that the food of the Adapinae 

 was of tougher fiber, requiring great muscular power of the jaws, exerted more vertically than in the 

 Notharctinse. 



In Adapis rutimeyeri (1916, Taf. xxi, fig. 24) p4 suggests that of A. magnus, but has not yet acquired 

 a metacristid; the entoconid is low, the protoconid-metaconid crest is more transverse. In Adapis 

 priscus (1916, p. 1511) p4 is nearly molariform with a well defined trigonid and talonid, including a fairly 

 large entoconid. The tooth is not compressed and elongate as it is in the speciahzed A. parisiensis. 

 It differs from p4 in all the Notharctina? in the relatively large size and advanced condition of the talonid, 

 including the hypoconid and the entoconid. 



