198 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



(3) The dentition of the Lemuridse is for the most part a degenerate derivative of that of the most 

 primitive Adapinge such as Adapis sciureus. 



(4) The "lemurine speciahzation" of the lower incisors and canines is correlated with the special 

 development of the tongue. 



Origin of the Peculiar Front Teeth (" Vordergebiss ") of Lemurs. Evolutionary Processes Illustrated 



Plates LII-LVIII 



The upper incisors of A^othardus and its allies are not reduced as they are in most Lemurida?; the 

 central upper incisors have a compressed oval crown with a more or less elongate edge, the crown being 

 supported by a long cylindrical procumbent root. The lateral upper incisors are much smaller, the 

 crown convex externally, obtusely pointed, flattened or concave on the lingual faces. The upper incisors 

 of Adapis magnus (Stehlin, 1916, pp. 1254, 12.55) were fundamentally similar but the central incisors 

 were wider. In Chirogaleus furcifer somewhat similar upper incisors persist although the medial pair 

 are separated by a considerable diastema. The separation of the opposite incisors, the straightening 

 of the opposite tooth rows and the truncate form of the premaxillse are probably all connected with the 

 great size and protrusile character of the tongue in lemurs. The upper incisors in Notharctinse and 

 Adapinse surrounded the end of the tongue, which was doubtless not yet so thick and protrusile as it is 

 in the Lemuridse. The lingual surfaces of the upper incisors, with their pits and ridges, were doubtless 

 more nearly related functionally with the tongue than with the lower teeth. The lower incisors have 

 short-crowned, more or less truncate or spatulate crowns, whereas in existing lemurs the incisors are 

 extremely long, compressed, styliform, and very sharply procumbent. 



The upper canines in Notharctus males are round in section and subcaniniform, whereas in lemurs 

 they are compressed with a very sharp posterior edge. The lower canines of Notharctus and Pelycodus 

 males are caniniform; in females they have shorter crowns and an internal cingulum, so that they are 

 somewhat premolariform; in Adapis rutimeyeri (?female, Stehlin, 1912, p. 1268, fig.) the small canine 

 has a short crown and a strong internal cingulum; this apparently approaches the type of canine, inter- 

 mediate in form between the lateral incisors and the first premolars, from which Dr. Stehlin believes 

 (1916, p. 1531) that the canines, not only of Lemur but also those of Adapis and Notharctus, have been 

 derived. In lemurs the lower canines have been taken over into the incisor series and are long, com- 

 pressed and styliform, differing from the incisors chiefly in their greater size and width. Thus the lower 

 canines of lemurs do not wear against the upper canines as in the normal mammalian dentition, but, 

 reaching forward to the anterior end of the jaw, they lie below the two upper incisors from which they 

 are more or less separated by the massive tongue; whereas in the Notharctinse the lower canines, being 

 erect and caniniform, do not reach the upper incisors but articulate in front of the upper canines, as in 

 primitive mammals. 



The first lower premolar of both Notharctinse. and Adapinse is small, simple and nearly erect; 

 it articulates between the first and second upper premolars; in the lemurs this tooth has disappeared 

 entirely, as well as the first upper premolar. 



The SECOND lower premolar of Notharctinse is likewise very simple and premolariform; it articu- 

 lates between the second and third upper premolars. In the lemurs, on the other hand, the anterior 

 lower premolar, which is homologous with the second lower premolar of Notharctus, is a large, compressed, 

 triangular, sharp-edged, cutting, piercing tooth, which at first sight looks like the lower canine; but it 

 articulates behind the upper canine like a premolar and not in front of the upper canine like a true lower 

 canine. 



