THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
549 
trally with the r.m.v.p.med., in which region also it show traces of myocommata. 
The latter disappear entirely in the dorsal area of this muscle. The uppermost end- 
line of the fibres, though arc-shaped, is less convex, and at its nearest point only 
extends within 19 mm. of the lateral line. In the region of insertion the superficial 
fibres of this muscle pass clear of the girdle, external to the scapula, to blend with 
those of the superficial constrictor muscles. This condition recalls that which is 
described by Humphry for Mustelus, and which obtains in numerous other shark-like 
Selachians ; in Chimsera this junction does not take place. 
(ii) R.m.v.p. medius. 
As in Selachians, this muscle is formed by the entire middle section of the mesio- 
ventral portion of the lateral muscle where it comes in contact with the girdle. The 
insertion is upon the inner aspect of the posterior rims of the scapula and coracoid 
from the ventralmost insertion of the r.m.v.p. sup. (and, in Chimsera, for some distance 
above and internal to that insertion) to the ventro-lateral angle of the body-wall. 
The muscle shows no tendency to spread upon the fin-muscles. The fibres run 
obliquely upwards from their insertions, and are intercepted a short distance behind 
the girdle by myocommata. 
(iii) R.m.v.p. inferior. 
This muscle only appears on the ventral aspect, and its fibres, inserted on the 
posterior face of the coracoid, run directly backwards parallel to the major body-axis. 
It is formed by the entire lower section of the mesio-ventral portion of the lateral 
muscle, which remains undifferentiated right up to its point of insertion. 
(3) The Anterior Muscles. 
In the anterior region we again encounter in the Holocephali an advanced state 
of evolution of the muscles which work the pectoral girdle. The muscles will be 
described in the following order, homologies with those of Selachians being implied 
by the use of identical terms : — 
{a) Protractor dorsalis pectoralis. 
(6) Muscles derived from the mesio-ventral portion of the lateral muscle : 
(i) Levator pectoralis. 
(ii) Coraco-branchiales. 
(iii) Coraco-hyoideus. 
(iv) Coraco-mandibularis. 
(a) Protractor dorsalis 'pectoralis ( p.d.p .) — It has already been seen that a large 
sheet of muscle derived from the entire latero-dorsal portion and to a certain extent 
from the mesio-dorsal portion of the lateral muscle is inserted on the posterior 
border of the uppermost section of the scapula ; this was designated the retractor 
dorsalis pectoralis. Another muscle, of precisely similar extent, bestrides the 
