MAY, 1921. AMERICAN MARSUPIAL, C^NOLESTES OSGOOD. 45 



extensors are reported to be fused at least proximally, but Parsons (1896, 

 p. 697) expresses the belief that they are normally separate in the 

 kangaroos. 



Extensor digitorum communis. Origin from the distal part of the 

 outer epicondylar ridge, the outer condyle and from the septa of the 

 muscles lying on either side, namely the extensor carpi radialis brevis and 

 the extensor digitorum lateralis. The fleshy part divides into two slips, 

 the more superficial of which sends tendons to the second and third 

 digits and the deeper one to the fourth and fifth. The tendons are 

 separate but closely parallel until they pass under the transverse liga- 

 ment and separate to insertions on the terminal phalanges of the second 

 to the fifth digits. The tendons become somewhat flattened or aponeu- 

 rotic distally. 



Extensor digitorum lateralis. Origin on the front of the outer con- 

 dyle of the humerus next to the extensor communis but quite distinct 

 from it. Connected by septa with the extensor carpi ulnaris. The head 

 of the radius lies immediately entad of the line of separation between 

 the two muscles. In the distal two thirds of its fleshy extent it is divided 

 into three distinct slips from which separate parallel tendons run one to 

 the fourth and two to the fifth digit. On the fourth digit the insertion 

 is at the base of the ungual phalanx. On the fifth the insertion in one 

 case is at the base of the ungual phalanx and in the other in the fascia 

 covering the second and third phalanges. A double tendon from this 

 muscle to the fifth digit has been noted in Sarcophilus (Macalister, 1870, 

 p. 164), but not in other marsupials. 



Extensor carpi ulnaris. Origin from the front of the outer condyle 

 of the humerus and the adjacent side of the ulna; also connected by 

 septa with the extensor digitorum lateralis. Its tendon passes over a 

 groove on the antero-internal surface of the extremity of the radius and 

 inserts at the base of the fifth metacarpal. According to Macalister 

 (1. c.) and Wilson (1894, p. 451), this muscle has no ulnar origin in 

 Sarcophilus, Didelphis, Trichosurus, and Dasyurus, but like Canolestes, 

 has one in Macropus, Phascolomys, Phascolarctos, and Perameks. 



Supinator (brevis). Origin tendinous on the sesamoid laterad of the 

 head of the radius. Insertion on the arched anterior surface of the prox- 

 imal third of the radius. A broad flat muscle. It occupies the upper two- 

 thirds of the radius in Phascolomys, two-fifths in Phascolarctos, one- 

 third in Macropus and Sarcophilus and only one-fourth in Didelphis and 

 Dasyurus (Macalister 1. c.). The presence of a sesamoid in its tendon of 

 origin appears to be unique among marsupials. It is reported in certain 

 edentates and bats and an origin from the orbicular ligament of the 

 radius was found in Dasyurus by MacCormick (fide Wilson, 1. c., p. 48). 



