OBSERVATIONS OF THE AUTHOR 428 



case of Notharctus the constriction between the two inner cusps marks 

 the spot where the entoconid of tlie lower molar sweeps across the in- 

 ternal ridge of the upper molar. In Notharctus, as in man}^ Perisso- 

 dactvls, the progressive development of the tetartocone, or posterointernal 

 division of the ])rotocone, is correlated with the progressive developjnent 

 of the entoconid of the lower molars. The progressive development of 

 the mesostyle is also correlated with a partly transverse excursion of the 

 mandible and with the Y-like modification of the para- and metacones. 



In the Adapida^, on the other hand, the entoconids, for some reason, 

 remained small ; there was consequently no correlated development of 

 the tetartocone, and the posterior cingulum was thus free to grow \^v in 

 a normal manner into a true hypocone, which fits into the valley of ihe 

 trigonid. Also, there being less transverse movement of the mandilde, 

 the para- and metacones did not become V-shaped and the mesostyle 

 failed to dcveloji. But, although these differences in the dentition are 

 very marked in the later members of the Adapinse and Notharctina\ 

 tliey ai'e less pronounced if we compare Stehlin's Protoadapis with our 

 American Pelycodus. The oldest forms of Pelycodus, which have recently 

 been described ])y Doctor Matthew,^ have extremely primitive trituber- 

 cular u])]ier molars, without any posterointernal cusp, and they have a 

 pattern which, according to accepted principles of dental evolution, is 

 structurally ancestral to the two divergent lines seen in the Notharctinae 

 and Adapina. 



Stehlin (1912), noting the marked differences in the dentition between 

 Adapis and Notli(ii'< hr;, l)ut without investigating the functional signifi- 

 cance of these differences, concluded that the Adapida? and the Xoth- 

 arctidse were rather \\i(h'ly separated families, not more nearly related 

 to each other than to oflicr groups of lemuroids ; but if Doctor Stehlin 

 had realized that these observed differences were all correlated with a 

 divergent habit of swinging the mandible, and that the more primitive 

 Notharctids of the Inwer Eocene appear to be structurally ancestral, both 

 to the Adaiuda' and Xothai('tida\ and ('S]iocially if he had had a well- 

 preserved skull of Xo/liarcl lis, and could ]ia\c seen the fundamental simi- 

 larity tlirouglioui. I think it probable that be would have been led to the 

 conclusion that the two gi'on|)> are lather nearly related. 



In Aihijris we see the same dental rorninhi as in the N'otharctintX!, but 

 the i-amus is nnw stout and the region of the angle is nnieh expanded, 

 paralleling in this respect such advanced lemuroids as the Indrisinje. In 

 fact, the dill'eiences in the lowci- jaw l)etween Notharctus and Adapis are 

 not as great as those ht.'lween Lcpilriinir of the Domurina^ and Indris of 



"Bull. Amer. Mus. Nat. Hist., vol. xxxlv, 1!)15, pp. 429-483. 



