GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



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are never distinct, the external cingulum is often obsolete, the internal cingulum is either much reduced 

 or very strongly developed, the hypocone is absent. So also in general contours of the molars a wide 

 adaptive radiation is shown: in Myoxicehus, which feeds on the leaves of the bamboo,^ the molars have 

 greatly expanded protocones and low conical para- and metacones, the protoloph is blunt and the surface 

 of the crown is slightly wrinkled; in Lemur, which is practically omnivorous, the molars are more dog- 

 like, with sharp protoloph and prominent internal cingulum. In Myoxicebus there is also an evident 

 tendency, already manifested in Adapts niagnus, for all the cheek teeth from p'^ to m^ to assume a similar 

 form with massive conical cusps. In Chirogale and Atililemur the molars are roundly tricuspidate, this 

 giving a " tritubercular " pattern of omnivorous type which is primitive in name only, but is in reality 

 degenerate and secondarily simple. In Microcebus the molars are small and delicate with pointed cusps, 

 indicative of a partly insectivorous diet. The molars of Lepile7nur are less degenerate in form than those 

 of any other genus except perhaps Mixocehm. They are in general similar in outline to those of Adapis 

 parisiensis as observed by Stehlin, save that they lack the hypocone; he, however, is unwilling to accept 

 this as a sign of close relationship. 



In general, the upper molars of the Lemuridte may be grouped under two main types: 



(1) the primitive sharp-cusped type with sharp cusps and cutting ridges recalling Adapis parisiensis; 

 Mixocebus, Lepilemur; the molars of Lemur and Microcebus are very specialized derivatives 

 of this type; 



(2) the round type, with very obtusely conical cusps, 



a) with distinct hypocone: Myoxicebus, 



b) hypocone reduced or wanting: Chirogaleus, Atililemur. 



As in the Adapinse there is never any tendency to form either a mesostyle or a pseudohypocone ; a 

 cingulum-hypocone and the protoloph crest are primitively well developed. 



The lower molars of the earlier species of Notharctinse are of an exceedingly primitive tuberculo- 

 sectorial type, common in its general pattern to many Paleocene and Eocene mammals and structurally 

 approaching the ancestral lower molar patterns of all other primates; they have a small trigonid, retain- 

 ing the primitive paraconid, and the third molar has a well developed hypoconulid; nia is much nar- 

 rower than m2; perhaps the chief specialized feature is the precocious development of the entoconid 

 which attains a large size in the later members of this subfamily. In Adapis parisiensis, typical of the 

 Adapinse, the molars are on the whole considerably more specialized in type. The paraconids are absent; 

 the protolophid crest is prominent and oblique; a metacristid is often present; the hypoconulid of nis 

 is smaller; nis is sometimes nearly as wide as m2. In the Lemuridee the lower molar patterns, though 

 more or less degenerate, are much closer to the Adapine than to the Notharctine type — Lepilemur in 

 fact retains much of the Adapis parisiensis pattern: the metacristid has become much enlarged and 

 displaced backward, usurping the position of an entoconid, for which it might readily be mistaken; 

 behind this enlarged metacristid is a notch representing the medial inlet to the talonid basin, and behind 

 the notch is the vestigial or absent entoconid and the crista posterior; the hypoconulid of ms though small 

 is present. In the highly specialized genus Lemur the cusps of the lower molars nii m2 have lost much 

 of their pristine and Eocene distinctness and are merged with the crests (protolophid, crista obliqua, 

 crista posterior). The metacristid crest is now very long and simulates a high sharp internal cingulum, 

 a structure which is never present in primitive Eocene molars; comparison with Lepilemur and Adapis 

 readily clears up the homologies. The medial inlet to the hypoconulid is now posterointernal in position;. 



1 Elliot, D. G., 1912, A Review of the Primates, I, p. 127. 



