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Show 268 MR. F. G. PARSONS ON T H E [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 extensive in the Hystricomorpha than in the Sciuromorpha. In Aulacodus 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 Hystrix, Sphingurus, Myopotamus, Octodon, and Ceredon to the lower half. In Castor to the middle third. In the other Sciuromorpha to the lower third. Supinator Longus.-This muscle is present in the Dipodidae, some of tbe Hystricidae, and the Sciuromorpha except Castor. In the Dipodidae, as in all the animals in which the muscle was found, the origin is from the external supracondylar ridge; the insertion, however, instead of being normal, is into the base of the metacarpal bone of the pollex (D. cegyptius, D. hirtipes, Alectaga indica). In the Hystricidae it is absent in Hystrix cristata and Sphingurus, but present in Erethizon dorsatus \ and, in a rudimentary condition, in E. epixanthus2; its attachments are normal. In the Sciuromorpha the muscle is well marked and the attachments normal; as above mentioned, it is absent in Castor. Extensor Carpi Radialis Longior and Brevior.-These muscles are always present, and only differ from the same muscles in M an 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 tbe same size except in Myopotamus, in which the brevis is much the larger and rises from a more extensive origin than its neighbour. Extensor Communis Digitorum.-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 vincula, which in Myopotamus are broad and membranous. In Dasyprocta the muscle rises from the upper two-thirds of the posterior surface of the shaft of the tdna 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 tbe 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 found3. 1 Mivart, P.Z. S. 1882, p. 271. 2 Windle, Journ. Anat. vol. xxii. p. 126. 3 P. Z. S. 1866, p. 383. |
