264 MR. r. G. PARSONS OK THE [Mar, 20, 



Wood ' are well illustrated ia the Eodents, though all three parts 

 are seldom present together. They all rise from the tip of the 

 coracoid process — the tirst part (rotator humeri) being inserted 

 into the surgical neck of the humerus above the insertion of the 

 latissimus dorsi tendon, the second part into the middle of the 

 shaft of the humerus, while the third part runs down to the 

 internal condyle. The musculo-cutaueous nerve always passes 

 between the first and second parts when these are present. 



In the Dipodidae the first and third heads are present in D. agyp- 

 iius and D. hirtipes, but in Alectaga apparently the second only. 



In the Octodontidte only the second head is present (Aulacodus, 

 Octodon, Capromys pilorides and melanurus ^, Myopotamus). 



In the Hystricidse only the second part is present in Hystrix, 

 while in Splimgurus, Eretliizon dorsatus^ and E. epixanthus^ the 

 second and third heads are found. The third head in Sphingurus 

 differed from the same part in the other animals I dissected in 

 having the median nerve separating it from the rest of the muscle. 



In the Chinchillidae only the second head is present in CJmichilla. 

 In Lagostomus the muscle was entirely absent on both sides, but 

 possibly this specimen was abnormal. 



In the DasyproctidsB the first and second parts are present 

 {Dasyprocta, Coelogenys). 



In the Caviidse only the second head is found (Oavia cobaya, 

 Ceredon rupestris, Dolkhotis '). 



In the Scinromorpha the rotator humeri is always present. In 

 Sciurus, Pteromys, Xerus, and Spermopliilus all three parts are 

 found, but the second and third are blended. In Arctomys the 

 first and second are present, while in Castor apparently the first 

 and third are found. The lower part in this animal is insei-ted by 

 a narrow tendon just above the inner condyle. 



Brachialis Anticus. — This muscle generally consists of an external 

 and an internal part. The external rises from the back of the neck 

 of the humerus and winds round to the front, lying just external to 

 the pectoral ridge, which, when it is well marked as in Aulacodus, has 

 a broad shallow groove for it to lie in. The internal head, when 

 it is present, is much smaller and rises from the anterior border 

 of the humerus below the pectoral ridge. The two parts are 

 inserted into the ulna just below the lesser sigmoid cavity, a 

 smaller shp being often sent to below and behind the tubercle of 

 the radius. The muscle is supplied by the musculo-cutaneous 

 and musculo-spiral nerves, but I was unable to satisfy myself that 

 each head had a different nerve-supply. 



In the Octodontidae both heads are present. 



In the Hystricidse, Eystrix has both heads, while Sphingurus 

 only has the external. 



1 Journ. Anat. vol. i. p. 45. 



2 Dobson, P. Z. S. 1884, p. 234. 



3 Mirart, P. Z. S. 1882, p. 271. 



■* Windle, Journ. Anat. vol. xxii. p. 126. 

 5 Beddard, P. Z. S. 1891, p. 236. 



