1894.] MYOLOGY OF RODENTS. 265 



In the Chinchillidae, Chinchilla has one head, Lagostomus two. 



In the Dasyproctidse both heads are found in Dasyproeta and 

 Cceloc/enys, although the inner head in the latter is very small 

 and blended with the outer. 



In the Caviidae only the outer head is found in Ceredon and 

 Cavia cobaya, but in Dolichotis, according to Beddard l , both parts 

 are present. 



In the Sciuromorpha both heads appear to be present, but they 

 are so closely blended as to be indistinguishable (Seiurus. Pteromys, 

 Xerus, Spermophilus, Castor). In Arctomys, however, no trace 

 of the inner head was seen. 



Triceps and Anconeus. — The triceps consists of the usual three 

 heads. The external head has a small origin from the back of the 

 neck of the humerus just above that of the brachialis anticus. 

 The middle or long head rises from a large part of the humeral 

 end of the axillary border of the scapula. The inner head rises 

 from the greater part of the posterior surface of the humerus and 

 is continuous with the anconeus. The insertion is into the 

 posterior part of the upper surface of the olecranon, the internal 

 head usually being inserted separately in front of the other two ; 

 the anconeus is attached to the outer side of the process. 



The triceps showed little variation in the different animals 

 examined ; in the Beaver it is well developed and attached to both 

 sides of the olecranon as well as to the top ; the anconeus is 

 especially well marked and rises from the enormous external 

 supracondylar ridge, it is inseparable from the inner head of 

 the triceps. 



Epitrochleo-anconeus. — In all the Rodents examined a small 

 round fleshy muscle rises from the internal condyle of the humerus 

 and is inserted into the inner side of the olecranon process, 

 covering the ulnar nerve. In Castor it is specially well developed. 



Pronator Radii Teres. — This muscle rises from the internal 

 condyle of the humerus ; it never has a deep head from the ulna, 

 and the median nerve always lies deep to it. It is inserted into 

 the convexity of the radius, usually about the middle. In Seiurus 

 and Pteromys, which possess a supracondylar foramen, the muscle 

 rises from the arch of bone forming it. 



In Aulacodus, HystrLv, Erethizon dorsatus 2 , Arctomys, and 

 Xerus the muscle is inserted near the distal end of the radius. 

 In all the other animals examined its insertion, as above stated, 

 was into the middle. 



In the Agouti I did not see the continuation of this muscle to 

 the carpus described by Mivart and Murie 3 . 



Flexor Carpi liadialis. — This muscle presented the usual 

 human attachments and relations in all the animals examined. 



Palmaris Lonyus. — The palmaris longus rises from the internal 

 condyle, and is inserted into the ulnar cartilaginous disk which 



1 P. Z. S. 1891, p. 236. 



2 P. Z. 8. L882, p. 271. 

 :i P. Z. S. 1866. p. 383 



