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Show 260 MR. P. G. PARSONS ON T H E [Mar. 20, In Octodon, a and B rise respectively from the anterior and posterior halves of the sternum and are not so closely united. In the Hystricidae a forms a separate band which runs obliquely across the rest of the muscle to be inserted quite at the lower half of the humerus, some fibres passing to the fascia of the forearm, y and 3 are inserted with B ; so that this family is remarkable for having the insertion of the pectoral almost entirely into the humerus (Hystrix, Sphingurus, Erethizon dorsatus1). The ChiuchillidaB resemble the Octodontidae in having a and B fused. <5 in the Chinchilla is inserted into the outer part of the clavicle, but in the Viscacha it is inserted into the coracoid process and first rib external to the origin of the subclavius. In the Dasyproctidae a is a distinct oblique slip as in the Hystricidae. y is inserted into the upper extremity of the humerus. B in Dasyprocta goes to the outer part of the clavicle blending with the sternoscapular. In Ccelogenys this part was not seen. This description differs from that of Mivart and Murie 2 in classing part of their pectoralis as deltoid. I find that the portion in question is supplied by the circumflex nerve and not by the anterior thoracic; as the circumflex also supplies the deltoid it is probable that the slip belongs to that muscle instead of to the pectoralis. (See Deltoid.) In the Caviidae there are no special fibres rising from the costal cartilages (Cavia cobaya, Ceredon rupestris). In the Sciuromorpha a has the usual origin and is inserted about the middle of the humerus. B rises from the whole of the sternum and runs almost horizontally to the whole of the pectoral ridge. y joins h at its insertion. S rises from the 2nd, 3rd, and 4th cartilages in Sciurus and Pteromys; from 2nd, 3rd, 4th, 5th, and 6th in Arctomys and Spermophilus. The insertion is into the shoulder-capsule and the coracoid process. Sterno-scapulari*.-This muscle consists of two parts which display a good deal of variety in different members of the group. The internal part or subclavius rises from the first rib at its junction with the sternum and is inserted into the posterior surface of the outer third of the clavicle. The outer part or scapulo-clavicularis, when it is present, rises from the clavicle close to the insertion of the subclavius, with which it is usually more or less continuous, and is inserted into the spine and vertebral border of the scapula, covering the supraspinatus as a broad thin sheet. In the Dipodidae the scapulo-clavicularis is absent but the subclavius is well marked (D. ceggptius, D. hirtipes, Alectaga indica). In the Octodontidae the two parts of the muscle communicate very slightly if at all in Octodon, Myopotamus, Capromys pilorides and C. melanurus3, but in Aulacodus many fibres are continuous. In the Hystricidae the two parts are continuous in Hystrix, but quite separate in Sphingurus. 1 P.Z.S. 1882, p. 281. 2 P.Z.S. 1866, p. 383. 3 Dobson, P. Z. S. 1884, p. 234. |