278 MB. F. G. PABSOKS ON the [Mar. 20 v , 



ribs from about the 9th to the last : there are thus two distinct 

 layers of fibres running in opposite directions in the dorsal region. 

 The variations that are met with consist of more or less complete 

 suppression of these parts. In Dasyprocta and Cavia cobaya, for 

 example, the posterior part is wanting and the anterior well 

 developed, so that in the former there is a continuous layer of 

 muscle, the fibres of which run in the same direction, stretching 

 from the 4th to the 13th rib. 



In Sphingurus, on the other hand, each part is equally diminished, 

 so that there is a space between them resembling the arrangement 

 in Man. 



In the Dipodidae the muscle is almost entirely represented by 

 fascia. 



Among the other animals examined Ceredon and Pteromys 

 resembled Aidacodus, while Ccelogenys, Arctomys, and Xerus had 

 the arrangement found in Dasyprocta. Octodon resembled Sphin- 

 gurus, but was remarkable for having the posterior part of the 

 muscle better developed than the anterior. 



Sacro-lumbalis. — This muscle has the usual attachments. It is 

 continued forwards by the accessorius, the limits of which are 

 indistinguishable. This is succeeded by the cervicalis ascendens, 

 which is attached to the transverse processes of the posterior three 

 cervical vertebra?, except in Dasyprocta and Coelogenys, where it 

 only goes to the last two. 



Longissimvs Dorsi. — This muscle, as well as the semispinalis and 

 multifidus, has the usual arrangement : their exact attachments 

 vary with the number of vertebrae. 



Transversalis Capitis and Colli. — "When both these muscles are 

 present they are continuous. The latter is attached to the 

 transverse processes, except sometimes the iirst and often the last 

 one or two. The transversalis capitis or trachelo-mastoid is 

 attached to the base of the paroccipital process, except in Castor, 

 where it goes to the base of the mastoid process. It is present in 

 all the Sciuromorpha, as well as in the Octodontidae, Hystricidae, 

 and Dasyproctidae. In the Caviidse it is present in Ceredon, but 

 absent in Cavia cobaya. In the remaining families the muscle was 

 not examined. 



Splenius Capitis et Colli. — The splenius capitis is always 

 present, and has the human attachments and relations. 



The splenius colli was found in the Dasyproctidae, where it was 

 inserted into the anterior three transverse processes in Dasyprocta 

 and into the transverse process of the atlas only in Coelogenys. 



A small slip representing this muscle was found in Myopotamus, 

 but in no other animal was it seen. 



Complexus. — This muscle has the usual attachments. It shows 

 signs of being divided longitudinally into two parts ; of these the 

 outer is inserted by tendon and the inner by flesh. In some of 

 the Hystricomorpha a slight tendinous intersection was seen in the 

 inner part, reminding one of the biventer of Man ; but this 

 arrangement was not seen in the Sciuromorpha, except in Castor, 



