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Show 1894.] MYOLOGY OP RODENTS. 265 In the Chinchillidae, Chinchilla has one head, Lagostomus two. In the Dasyproctidae both heads are found in Dasgprocta and Ccelogenys, 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 1, both parts are present. In the Sciuromorpha both heads appear to be present, but they are so closely blended as to be indistinguishable (Sciurus. 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 tbe 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 Bodents 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 Sciurus and Pteromys, which possess a supracondylar foramen, the muscle rises from the arch of bone forming it. In Aulacodus, Hystrix, Erethizon dorsatus2, 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 \ Flexor Carpi Radudis.-This muscle presented the usual human attachments and relations in all the animals examined. Palmaris Longus.-The palmaris longus rises from the internal condyle, and is inserted into the ulnar cartilaginous disk which i P. Z. S. 1891, p. 236. 2 P. Z. S. 1882, p. 271. 3 P. Z. S. 1866. p. 383 |