78 



GREGORY: NOTHARCTUS, AN AMERICAN EOCENE PRIMATE 



(5) Among the Lemuriformes there is a steady rise in the humeral index, as we pass from the more 

 primitive forms with wider humeri to the speciaUzed lemurs with long humeri. 



(6) In the brachiating series the index rises rapidly from 49 in the Cebidse to 84 in Hylohates, which 

 has the longest humerus of the series. In Hapale the humerus is perhaps secondarily widened 

 through the use of the claws in climbing. 



Radius 

 Text Fig-. 10 



This element is represented by the right radius and the distal end of the left radius of A^. osborni, 

 Amer. Mus. No. 11474, by the right radius and portion of the left radius of N. tyrannus, Amer. Mus. No. 

 11478, as well as by other specimens. Resemblance to the radius of Lepilemur is extremely close and 

 detailed. The principal difference is that the shaft is wider, especially at the lower end. and there is no 



subscapularis 

 teres major 

 latissimus dorsi 

 biceps 



brachials anticus 



Fig. 9. Arm muscles of Ijinvr 



q coracobrachialis 

 t, r-, P triceps 



anconeus internus 

 V supinator longus 

 X pronator radii teres 



After r'u\ iei 



Laurilh 



extensor carpi 



radialis longior 

 flexor profundus digitorum 

 flexor carpi radialis 

 flexor sublimis digitorum 



a jialmaris 



flexor carpi unlaris 

 $ abductor pollicis brevis 

 p adductor pollicis 

 R annular ligament 



groove for the tendon of the extensor ossis metacarpi pollicis, such as is present in Lepilemur and Lemur. 

 About the middle of the shaft on the anterior border there is a gentle eminence to which the tendon of 

 the pronator radii teres ^ was very probably attached; this eminence is better developed than in recent 



1 Dr. Wood Jones (1916, p. 38) notes that the pronator radii teres has but a single head (arising from the upper surface of the 

 internal epicondyle of the humerus) in lemurs, in monkeys, and in most placental orders. On the other hand, a second head is present 

 (arising from the coronoid process of the ulna) in primitive amphibians and reptiles, in Tupaia and Crocidura, in most chimpanzees, 

 in many gorillas, in the orang, and usually in man. He therefore regards the bicipital condition of the pronator radii teres as primitive. 



While it may possibly be true that man and the anthropoids retain a primitive muscle slip which has been lost in the lemurs, 

 it is practically certain that, with regard to the form of the humerus, of the radius, and of the ulna, the lemurs as a whole are much 

 more primitive than the anthropoid-man group. Dr. Wood Jones (p. 44) speaks of "the primitive type of every bons and joint of the 

 human fore-limb.'' In view of the fact that Eocene and even modern lemurs retain very many characters in humerus, radius, and ulna, 

 which are found in other quadrupedal Eocene mammals, but are absent in the semierect to erect man-anthropoid group, the phrase 

 nolcd above appears to be most misleading. (Cf. Plate XXVII.) 



