THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
539 
Now, it has been seen that the latter is absent in Mustelus (Humphry) and, at least, 
in some specimens of Scyllium, and in these cases the p.l.d.p. is also wanting. 
In Rhina, where the r.l.d.p. is well marked, the p.l.d.p. is also well developed, 
taking origin by a tendinous sheet from the knob-like extremity of the scapula. 
In Raia the anterior border of the cartilage which joins the scapula to the 
vertebral column is wider than the posterior border. For this reason all the fibres 
of the dorsal moiety of the lateral muscle which are attached anteriorly to the 
pectoral girdle take origin. on the said cartilage. 
(6) Protractor latero-ventralis pectoralis (p.l.v.p.). Protractor scapulae, Owen.— 
Actually the p.l.v.p. is overlain near its insertion by the levator pectoralis and, 
further forward, by the superficial branchial muscles ; so that only a very small 
portion of it is visible without dissection (at its insertion). In Scyllium, Galeus, 
Acanthias, and Rhina this muscle shows marked constancy, and the following 
description is applicable to any of these types. 
Immediately below the lateral line fibres are inserted into the outer and inner 
aspects of the anterior border of the scapula. Tracing these fibres forwards, after 
reflecting the overlying muscles noted above, they are found to take origin from the 
deeper fibres of the latero-ventral portion of the lateral muscle. Thenceforward the 
entire muscle is continued as a dorso-ventrally compressed mass which terminates 
in a tendinous insertion on the base of the chondrocranium. It is familiar to all 
who have .had occasion to dissect the Dogfish as that muscle which separates the 
lateral nerve from the anterior cardinal sinus. 
In Raia, though very much reduced, this muscle is present as a triangular strip, 
whose apex is inserted on the ventral aspect of the anterior upper portion, of the 
scapula, while its base takes origin along the outer rim of the great lateral process 
of the first vertebra. The muscle was described by Marion (62) as “ portion C of 
the trapezius.” 
Doubtless the great development of the vertebral process in Raia accounts for 
the fact that no part of this muscle reaches the cranium, which is the case in the 
shark-like forms. 
(c) Muscles derived from the mesio-ventral portion of the lateral muscle. 
(i) Levator pectoralis (l.p.). Levator scapulae, Humphry ; Trapezius (pars), 
Vetter, Gegenbaur, Marion. 
The levator pectoralis is a compact triangular muscle whose base runs along the 
lateral septum from just in front of the scapula to the level of the third branchial 
arch. Its apex is curved slightly forwards, so that the posterior side of the triangle 
is slightly convex and the anterior slightly concave. The posterior convex border 
is inserted into the outer face of the scapula, which is usually grooved (markedly in 
Acanthias) to receive it. The body of the l.p. forms the outer wall of the anterior 
cardinal sinus. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 21). 
83 
