greCxOry: notharctus, an American eocene primate 



203 



proved precedents for such a marked change in the direction of evohition in the dentition; he therefore 

 beUeves, until further evidence is produced, that the Law of Irreversibihty of Evohition here sets a hmit 

 to the possibihties of evolution. He will not admit that the canines of any group of Primates were origi- 

 nally caniniform, but holds that they were brachyodont and structurally intermediate between the ante- 

 rior premolars and the lateral incisors. From such a ground-i)lau the canine of Lemur may i)e as easily 

 derived as those of Adapis and Notharctus (p. 1531). 



The writer, on the other hand, feels that the Law of Irreversibility is not incompatible with marked 

 changes of function, of form and of the direction of evolution. He concludes that in the Lemuridie a 

 profound change of form and function in the lower incisors, canines and p^ has been conditioned in part 

 by the enlargement of the tongue in adaptation to frugi\'orous hal)its; that the ancestral Lemuridae 

 had the lower front teeth not dissimilar to those of Adopts sciureus, that is with subcaniniform canines 

 much larger than the first premolar; that all groups of Primates may eventually have been dei'ived from 

 small insectivorous-frugivorous arboreal mammals with subcaniniform canines. 



Premolars 

 Phites LIV, LV 



Except for the marked enlargement of p.. the premolar patterns in the modern Lemuridffi represent 

 for the most part degenerative derivatives of a primitive Adapine type. The upper premolars of the 

 Notharctinse differ from those of the Lemuridee as follows: 



(1) The Notharctinse as stated above retain pi which have been crowded out in the Lemuridae. The 

 loss of p^ provides a space for the reception of the tip of the enlarged po; the loss of pi permits po to gain 

 wide contact with the upper canine; when therefore diastemata are developed separating p- from p'*, 

 as in Microcebus, it is a sign that the elongation of the face which has produced the diastema is secondary. 



(2) The second upper premolar varies considerably in form in the Notharctinse. In A^. venticolus it 

 has two distinct roots and a compressed conical crown; in N. oshorni 9 the roots are almost connate and 

 the crown is smaller and less compressed; in the Adapina? the I'oots of p- also seem to be either united or 

 connate and the tooth is either compressed (A. iKirisiensis) or widened (A. nutgnus). So too in the 

 Lemuridae p- is either a small single-fanged tooth as in the Chirogalein-ce, or it has two roots and a large 

 compressed crown as in Lepilernur, Mixocebus and Myoxicebus. 



The second lower premolar, relatively small in the Notharctinse, is com])ressed in the typical Adapis 

 parisiensis, wider in A. magnus. In the very primitive A. sciureus it has an obliquely oval crown with 

 ah internal cingulum; it could readily be enlarged into the form characteristic of the Lemuridae which is 

 obliquely asymmetrical and directed forward and upward so as to engage with the posterior face of the 

 upper canine. 



The third upper premolar is distinctly premolariform and somewhat like p^ in all the Notharctinae 

 and in Adapis magnus and A. rutinieyeri. In A. parisiensis it is compressed and simple. So too, in the 

 Lemuridae, p^ is either much like p"*, as in Mixocebus, Myoxicebus, or it is peg-like and simple, as in .l/;7/- 

 lemur, Chirogale, Myoxicebus; or compressed conical with two widely separated external roots and with 

 only a slight internal extension, as in Lemur and Lepilernur. The internal extension of p"' also exhibits 

 a wide range in form; in Myoxicebus it is very large and massive; in Microcebus it is practically absent. 



A similar range of variatit)n is seen in the structure of p.^ in the Notharctina^, Adapina' and Lemuridae, 

 the whole crown being more or less compressed or widened, and the talonid either veiy slight or Ijetter 



