GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



79 



lemurs. The interosseous ridge, from which was stretched the interosseous membrane running to the 

 ulna, is well marked but not so sharp as it is in Lemur. The anterior border of the shaft is not nearly 

 so thick as it is in Lemur and Lepilemur; the styloid process was less produced downward and there 

 was no distinct groove for the tendon of the extensor ossis metacarpi poUicis. The process for the attach- 

 ment of the supinator longus was more prominent. The marked bowing of the radius and ulna in opposite 

 directions afforded space for robust extensor and flexor muscles of the carpus and poUex. 



12 3 4 



V\<^. 10. Comparative series: right radius ami rigiit ulna of yofhdrctii.f and Lciinir. Natural size. 



1. Noiharclus osborni. Amor. Mus. No. 11474. Inner, or flexor, side of forearm bones. 



2. Lemur mongoz. Amer. Mus. No. 22886. Inner, or flexor, side. 



3. 4. Outer, or extensor, side of same speeimens. 



The radius of Propithecus, except for its greater length, in all views is strikingly similar to that of 

 Notharctus; it is also curved anteroposteriorly as in Notharctus and is but little modified by the further 

 flattening of the distal end and the rounding of the shaft. As in Notharctus, there is a distinct process 

 on the anterior border near the lower end, for the tendon of the supinator longus, and both genera lack 

 the deep groove for the extensor ossis metacarpi poUicis which is seen in Lepilemur and Lemur. In Pro- 

 pithecus the groove for the extensor communis digitorum is converted into a tunnel beneath the dorsal 

 ligament. 



The radius of Cehus and Alouatta as compared with that of Notharctus is much longer and has 

 a cyhndrical shaft; distally it is much thicker in section; the region of the styloid process is more pro- 

 duced downward and inward. But only a small change, i. e., the conversion of a flattened shaft into a 

 cylindrical one, would be required to transform a A^otharctus-\\ke radius into the cebid type. 



