GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



225 



of incisors, canines and true molars, which are also nearly alike in constitution. Xothardiifi possesses one more premolar 

 and the others have a pair of fangs. The resemblance is so close that but little change would be necessary to evolve from 

 the jaw and teeth of Notharctus that of a modern monkey. The same condition which would lead to the suppression of a 

 first premolar, in continuance would reduce the fangs of the other premolars to a single one. This change, with a con- 

 comitant shortening and increase of depth of the jaw, would give the characters of the living Cehus. A further reduction 

 of a single premolar would give rise to the condition of the jaw in the Old World apes and man. 



Dentition 



The adult dental formula of Notharctus is 



(I| C{ P| Mi) X 2 = 40. 



This is undoubtedly the primitive formula for Primates and it differs from that of the most primitive 

 placental mammals only in the reduction of the incisor formula from if to l|. In later Primates this 

 formula suffers various reductions: as by the loss of some of the incisors (Indrisidse, Chiromyidse), of 

 the anterior premolars (pi) (most lemuroids, New World monkeys), of pr and p| (Indrisidse, Old World 

 monkeys, great apes, man). 



The central upper incisors of Notharctus have short, anteroposteriorly elongate, compressed crowns 

 and the lateral upper incisors are small and round-topped; the upper incisors are thus more primitive 

 in form than the chisel-like incisors of man and of the Old World and New World monkeys; a some- 

 what similar style of incisors has survived in the modern Chirogaleus. The lower incisors of Notharctus 

 are much less specialized than those of Lemur in their more or less spatulate form and wide cutting edge, 

 much as in the New World and Old World monkeys, whereas in Lemur, the lower incisors are degenerate, 

 procumbent, styliform, compressed. The upper and lower canines of Notharctus are caniniform, but of 

 a more primitive type than the variously modified canines of later primates. 



The first upper premolar of Notharctus is small and of little functional importance. The crown of 

 the second upper premolar consists of a single compressed cusp supported by an incipiently two-rooted 

 fang. In more recent lemurs the anterior upper premolar (p^) often becomes much compressed; in the 

 South American monkeys it becomes widened into a bicuspid tooth with a single external root; m the 

 Old World monkeys, great apes, and man the second upper premolar (p-) of the primiti\^e mammalian 

 dentition has been crowded out, and the first bicuspid tooth, immediately behind the canine, is p^ Here 

 again Notharctus is obviously more like other primitive placentals than are any of the later lemurs or 

 monkeys. 



The third upper premolar (p'O of Notharctus has an outer wall with two roots, a well-defined external 

 cingulum, a conspicuous external cusp, a small mesostyle or external intermediate cusp and a low narrow 

 internal cusp which is continuous with the internal cingulum. This primitive style of premolar, which 

 approaches that of Eocene Carnivores and other primitive placentals, is variously modified in the higher 

 primates, often becoming bicuspid, as in man. 



The fourth upper premolar (p"*) is considerably larger than p'* and is also more complicated, as is 

 generally the case in primitive Eocene mammals, the simpler pattern of the premolars in the higher 

 primates being wholly unlike that of primitive Eocene mammals and obviously retrogressive. P^ has 

 a large internal cusp, which is rounder and more symmetrical than the corresponding cusp of p^, its outer 

 wall is higher, flatter, more symmetrical, and in the higher species of Notharctus it is divided into two cusps. 

 This largely primitive type of p^ is variously transformed in the higher primates: in many lemurs, its 



