260 MS,. F. G. PABSOxs ON THE [Mar. 20, 



In Octodon, a and /3 rise respectively from the anterior and posterior 

 halves of the sternum and are not so closely united. 



In the Hystricidoe 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 fore- 

 arm, y and S are inserted with /3 ; so that this family is remarkable 

 for having the insertion of the pectoral almost entirely into the 

 humerus {Hystrix, Sphinr/urus, Erethizoii dorsatus^). 



The Chiuchillidae resemble the Octodontidae in ha\dng a and /3 

 fused. S 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 Dasyproctidse a is a distinct oblique slip as in the 

 Hystricidfe. y is inserted into the upper extremity of the 

 humerus, o 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 " 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 shp belongs to that muscle instead 

 of to the pectoralis. (See Deltoid.) 



In the- CaviiJae there are no special fibres rising from the costal 

 cartilages {Cavia cobaya, Ceredon rupestris). 



In the Sciuromorpha a lias the usual origin and is inserted about 

 the middle of the humerus. /3 rises from the whole of the sternum 

 and runs almost horizontally to the whole of the pectoral ridge. 

 y joins I at its insertion, h rises from the 2nd, 3rd, and 4th 

 cartilages in Scmrus 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 thnd 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 sub- 

 clavius is well marked {D. cegyptius, I), hirtipes, Alectaya indica). 



In the Octodontidae the two parts of the muscle communicate 

 very slightly if at all in Octodon, Myopotamus, Gapromys pilorides 

 and C. melanurus^, but in Aulacodiis many fibres are continuous. 



In the Hystricidse 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. 



