186 



GREGORY: 



AYJrifARCTUS, AN AMERICAN EOCENE PRIMATE 



outstanding difficulty is to settle the precise relationships of those Lower Bridger forms which are of 

 intermediate size; namely, A'', tyrannus, N. anceps, N. affinis. 



The evolution of the dentition as a whole appears to be as follows: 



In the earliest forms, Pelycodus ralstoni and P. trigonodus, the dentition although not exclusively 

 insectivorous in character retains many reminders of a generalized insectivorous type ; the animals them- 

 selves were of very small size; the incisors and canines were small, the lower incisors gently procumbent, 

 the premolars simple, the upper premolars tritul^ercular, the lower molars tuberculosectorial with low 

 entoconids; the dentition lacked only the third upper and lower incisors of the primitive insectivorous 

 Eutherian formula. The lower jaw in these primitive types was relatively slender and probably its motion 

 was largely orthal. The latest form, Nothardus crassus, was nearly as large as a howler monkey; the 

 canines are caniniform, with long thick fangs, the fourth and even the third premolars are becoming more 

 molariform, the upper molars have two external V's, a large mesostyle and a large pseudohypocone ; the 

 lower molars have the paraconids reduced or absent and now have large entoconids; all the cusps were 

 coarse and subcircular. The mandible could be displaced transversely from the outer side inward, this 

 giving considerable cutting and grinding action to the molars.' The lower incisors were smaller than 

 those of the Adapina3 with the crown less extended transversely; the central upper incisors were com- 

 pressed and inclined toward the midline; the lateral incisors were behind them. On the whole the 

 incisors were adapted rather for prehension than for cutting. Such a dentition seems to have been adapted 

 largely to a diet of leaves and soft-rind fruits. As in other families of primates in which the canines 

 progressively become caniniform the males of the robust species of Notharctina^ were probably more or 

 less aggressive fighters. 



The Adapinse range in size from Adapts sciureus (Stehlin, 1916, p. 151.5, fig.) which was smaller 

 than the smallest known Notharctine (nii-s measuring only about 9.3 mm., as compared with 11-14 in 

 Pelycodus ralstoni) up to Adapts magrms in which nii-a measures 23.5 (about the same as in Nothardus 

 crassus). In A. tnagnus, and still more in A. parisiensis, the very massive jaws and muscle supports 

 were much larger in proportion to the size of the cheek teeth than in any of the Notharctinse. The incisors, 

 canines and premolars all had sharp cutting edges; the incisors were wide and chisel-like, the canines 

 more or less premolarif orm ; the fourth premolars were almost molariform, the others more or less tren- 

 chant. The upper molars had a partly flattened ectoloph, rather delicate conical para- and metacones, 

 and a sharp oblique protoloph; there were no mesostyles and no pseudohypocones but true cingulum 

 hypocones. The low hypocones did not oppose the low entoconids, but jutted inward above the trigo- 

 nids of the lower molars. The lower molars lacked paraconids, and had low entoconids; they bore a 

 prominent oblique metalophid, ending lingually in a large metacristid and shearing past the protoloph 

 of the upper molars. The motion of the jaw was more orthal and less ental than in the Notharctinse, 

 and the condyles show corresponding differences. 



The dentition of the more specialized Adapinse presents certain analogies to those of ungulates, 

 especially Eocene perissodactyls with lophodont molars. On the other hand the persistence of the brachyo- 

 dont condition and the conical sharp character of the para- and metacones, together with the more 

 orthal movement of the mandible, suggest that the food was not ground but was merely pierced, cut, 

 and pressed. The closest resemblances to Adapts, however, both in the patterns of the premolars and 

 molars and in the interlocking relations between the upper and the lower teeth, are seen in the modern 



' This movement of the mandible from tlie outer side inward, pressure being exerted first on the outer side, is here called entad, 

 as in the Century Dictionary enlnl, by analogy with proal and palinal, notwithstanding that Cope (.lourn. of Morphol., 1889, p. 226) 

 gave to ental and ectal meanings which were precisely the reverse of what one would have expected from their etymology. 



