18(30".] ON THE ANATOMY OF THE (RESTED AGOUTI. 397 



Muscles of the Fore Limb. 



Pectoralis major. Perhaps the most striking difference between 

 the posterior fibres of this muscle and that of the conjoined panni- 

 culus carnosus is the darker colour of the former. The pectoralis 

 major is extensive, but on the whole of moderate thickness. Al- 

 though peculiar, we did not find it to answer the description given 

 by Prof. Owen of the same in Capromys fournieri* ; for it seemed 

 to have four distinct origins, and as many insertions. 



The first and largest portion consists of a very broad sheet of 

 muscular fibre arising from the posterior two-thirds of the sternum, 

 and, as already stated, is intimately blended with the panniculus 

 carnosus (see fig. 2, P. m. 1); while it is also much adherent to the 

 posterior portion of the sterno -scapular muscle. Its insertion extends 

 from halfway down the deltoid ridge, upwards as far as the tendon 

 of the coraco-brachialis ; and a small slip of its fibres passes over 

 this tendon to the surface of the fascia covering the subscapularis. 



The second portion arises from the anterior fifth of the sternum, 

 almost as far as the end of the manubrium. Its posterior border is 

 closely connected with the anterior border of the first portion, so that 

 these two together almost, if not quite, cover the sternal origin of 

 the sterno-scapular. It is inserted into the shaft of the humerus, its 

 whole length external to the insertion of the preceding portion (figs. 

 2 & 3, P. m. 2). 



The third portion is a narrow band of muscle arising from the 

 manubrium, and inserted into the lower end of the shaft of the hu- 

 merus, immediately external to the second portion of this muscle 

 (figs. 2 & 3, P. m. 3). 



The fourth portion is a still narrower band, which arises from 

 almost the outer end of the clavicle, and joining the third portion is 

 inserted in common with it ; and it is this junction and common in- 

 sertion with an undoubted part of the pectoralis major which decides 

 us to regard it as part of the pectoral, and not as a part of the 

 deltoid (figs. 2 & 3, P. m. 4). 



In the Hare, the Rabbit, and the Guinea-pig this muscle exists in 

 a slightly modified condition from the above ; and although the 

 several portions spoken of are in them more adherent, nevertheless 

 they are without much difficulty separable into nearly similar slips. 

 What answers to the first and second portions in D. cristata may in 

 them be regarded as but one sheet folded on itself at its sternal and 

 humeral attachments. The insertion into the humerus of the third 

 portion is neither so extensive nor so muscular in the three rodents 

 in question. 



In Dasyprocta cristata the pectoralis minor is wanting, or included 

 in the p. major ; the slip of the first portion of the latter muscle 

 going to the subscapularis, possibly representing the insertion of the 

 fused pectoralis minor. 



Meckel f, in his description of a small muscle underneath the 

 pectoralis major in the Marmot &c, says, " II est possible, au reste, 



* P. Z. S. 1832, p. 74. t Loc. dt. p. 201. 



