1894.] MYOLOGY OF RODENTS. 277 



occiput, from the ligarnentum nuchae, and from the anterior three 

 or four dorsal spines and supraspinous ligaments ; the insertion is 

 into the whole length of the vertebral border of the scapula, the 

 occipital fibres also going to the fascia over the inner part of the 

 supraspinatus. 



Sciurus and Pteromys differ in having a slight separation between 

 the occipital and cervical portions, but this is not seen in Xerus 

 and Spermophilus. 



Serratus Magnus and Levator Anguli Scapula?. — These two 

 muscles are in the same plane and usually have a continuous 

 origin, so that it is difficult to define their line of demarcation. 

 They rise from the transverse processes of several cervical vertebrae, 

 and from the sides of 7 to 9 of the anterior ribs by fleshy digita- 

 tions. The insertion is into the vertebral border of the scapula. 

 In the following animals a separate slip rising from the atlas was 

 found : — Myopotamus, Sphingurus, Lagostomus, Sciurus, Pteromys, 

 and Arctomys. 



The exact number of vertebrae and ribs from which these muscles 

 arise in various Rodents are as follows : — 



Dipus cegyptius 1-7 c. v. 1-8 ribs. 



Capromys pilorides 2—7 „ 1-9 „ 



„ melanurus 1 .... 3-7 „ 1-6 ,, 



Aulacodus 2-7 „ 1-9 „ 



Myopotamus 1st & 3-7 ,, 1-8 „ 



Octodon 5-7 „ 1-8 „ 



Hystrix 4-7 „ 1-8 „ 



Sphingurus 1—7 „ 1-7 „ 



Chinchilla 2-7 „ 1-9 „ 



Layostomus 1-7 „ 1-8 „ 



Basyprocta cristata 2-7 „ 1-8 „ 



Ccelogenys 1-7 „ 1—8 „ 



Cavia cobaya 1-7 ,, 1-9 ,, 



Ceredon .. 2-7 „ 1-9 „ 



Sciurus 1st & 3-7 „ 1—8 ,, 



Pteromys 1-7 „ 1-7 ,, 



Xerus 4-7 ,, 1-8 „ 



Arctomys marmotta . . 1st & 3-7 „ 1-9 „ 



Castor canadensis 2-7 ,, 1—7 „ 



Serratus Posticus. — This muscle varies very much in different 

 genera and apparently in different individuals. Aulacodus seems 

 to show most satisfactorily its full development. In this animal 

 the anterior part of the muscle rises from the ligarnentum nuchae 

 and spines of the anterior dorsal vertebra? to be inserted into the 

 ribs from about the 4th to the 12th, the direction of its fibres 

 being backwards and outwards. The posterior part rises from the 

 spines of the posterior dorsal and lumbar vertebrae by means of the 

 lumbar fascia, and runs forwards and outwards to the posterior 



1 Dobson, P. Z. S. 1884, p. 234. 

 Pboc. Zool. Soc— 1894, No. XIX. 19 



