THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
537 
fibres are attached to its inner aspect ; these constitute the retractor latero-ventralis 
pectoralis, and are distinguished by their peculiar dark colour. In actuality this 
muscle is more or less covered by the musfcle next to be considered — the r.m.v.p. 
superior. 
In the above types the muscle under discussion is divided by a fascia from the 
next of the series of retractors. In Rhina, on the contrary, there is no such distinction, 
the r.l.v.p. passing conformably into the r.m.v.p. 
In Raia this muscle is present as a thin ribbon attached to the postero-ventral 
aspect of the scapula external to the lateral nerve. It differs from the above types 
in that it is entirely interrupted by the scapula, no part of it running forward beneath 
that cartilage. 
(c) Retractor mesio-ventralis pectoralis (r.m.v.p .). — The mesio-ventral portion 
encounters in its upper reaches the fin, and in its lower reaches the coracoid portion 
of the pectoral girdle, and is interrupted, throughout its depth by these structures. 
It is more or less readily divisible into three regions of insertion : (i) ‘ superior — 
from the scapula and upper surface of the fin ; (ii) medius — from the axil of the fin ; 
and (iii) inferior — from the coracoid and under surface of the fin. 
(i) R.m.v.p. superior. Latissimus dorsi, Humphry ; Serratus, Braus (14). 
Humphry described this muscle in Mustelus as follows : It is inserted into the 
scapular part of the girdle, and expands upon the dorsal surface of the fin, reaching 
to its anterior edge. Upon the fin it lies upon, and to some extent blends with, the 
proper muscle of the fin.” 
I find that this condition obtains in Scyllium, Galeus, and Acanthias. The fibres 
run obliquely throughout the depth of the muscle upwards from their insertion ; the 
muscle never attains a great thickness. If we take Humphry’s description of the 
insertion, the origin is found to be, dorsally, from the surface of the r.l.v.p., while, 
ventrally, the fibres are continuous with those of the upper section of the mesio- 
ventral portion of the lateral muscle. Arching upwards, the end-line of the fibres 
extends as far up the side as the lateral line a short distance (about 10 mm. in 
Scyllium) behind the girdle. Posteriorly, the r.m.v.p.sup. rapidly diminishes. 
In Raia this muscle does not cover the surface of the fin, but finds its insertion 
along the upper posterior rim of the metapterygium, as well as upon the scapula. 
(ii) R.m.v.p. medius. Axillary portion, Humphry. 
That area of the mesio-ventral portion of the lateral muscle which meets the 
articular region of the pectoral fin is here implied. The fibres run obliquely in the 
same direction as those of the r.m.v.p.sup. The muscle is sometimes (Scyllium, 
G-aleus) separated at its insertion from the last-named by a patch of connective tissue, 
in which case the muscle appears to concentrate upon a limited area of the coracoid 
just in front of the glenoid border.. In Galeus and Mustelus (Humphry) the insertion 
is more extensive and spreads upon the axillary aspect of the fin muscles, both 
