268 SLR. F. G. PAEsoxs ON THE [Mar. 20, 



although, as a general rule, pronation is only allowed through 

 about one-eighth of a circle. The extent of the muscle varies from 

 one-third to the whole of the interosseous space, being much more 

 e.\teusive in the Hystricomorpha than in the Sciuromorpha. 



In Aidacodus and Dasyprocta it is attached to the whole length 

 of the contiguous margins of the radius and ulna. 



In Ccelogenys to the lower three-quarters. In Lagostomus to 

 the lower two-thirds. 



In Hystrir, Spliingurus, Myopotamus, Octodon, and Ceredon to 

 the lower half. 



la Castor to the middle third. In the other Sciuromorpha to 

 the lower third. 



Supinator Longus. — This muscle is present in the Dipodidse, 

 some of the Hystricidae, and the Sciuromorpha except Castor. 



In the Dipodidae, as in all the animals in which the muscle was 

 found, the origiu is from the external supracondylar ridge ; the 

 insertion, however, instead of being normal, is into the base of the 

 metacarpal bone of the pollex {JJ. cegyptius, D. Iiirtipes, Alectaga 

 indica). 



In the Hystricidae it is absent in HystrLv cristata and >S^:>7a>i- 

 gurus, but present in Erethizon d,orsatus \ and, in a rudimentary 

 condition, in E. epixanthus ^ ; its attachments are normal. In the 

 Sciuromorpha the muscle is well marked and the attachments 

 normal ; as above mentioned, it is absent in Castor. 



E.vtensor Carpi Eadialis Longior and Brevior.- — These muscles 

 are always present, and only differ from the same muscles in Man 

 in that they are attached to the middle of the shafts of the 

 metacarpal bones instead of near the bases. The two muscles are 

 about the same size except in 3Iyopotamus, in which the brevis is 

 much the larger and rises from a more extensive origin than 

 its neighbour. 



E.vtensor Communis 3igitorum. — This muscle rises from the 

 external condyle, and is inserted into the middle and distal 

 phalanges of the 2nd, 3rd, 4th, and 5th digits. On each side of 

 the slip to the terminal phalanx there is a strong elastic band 

 running from the head of the middle to the base of the distal 

 phalanx ; this serves to keep the terminal joint of the digit in a 

 state of constant extension. The four divisions to the fingers are 

 connected by viucula, which in Alyopotamiis are broad and 

 membranous. 



In Dasyprocta the muscle rises from the upper two-thirds of 

 the posterior surface of the shaft of the ulna as well as from the 

 external condyle ; it divides into three slips, of which the middle 

 goes to the 2nd, 3rd, and 4th digits, the radial one joins the 

 tendon of the middle part to the index, while the ulnar slip goes 

 to the 3rd and 5th digits. This practically corresponds to what 

 Mivart and Murie found ^. 



' Mivart, P. Z. S. 1882, p. 271. 



- Wiiidle, Joum. Anat. vol. xxii. p. 126. 



" P. Z. S. 1860, p. 383. 



