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CAPTAIN E. W. SHANN ON 
abductor and adductor, as well as upon the scapula (Galeus), in addition to the 
typical insertion. 
The Rays differ from the shark-like types in that in them this muscle is inserted 
along the whole length of the metapterygium, beneath the r.m.v.p.sup., as well as 
to the scapular and coracoid portions of the girdle. 
(iii) R.m.y.p. inferior. Pectoralis, Humphry ; Rectus (?). 
This muscle is confined to'the ventral aspect, and is very constant in form through- 
out the Selachii. In appearance it recalls the rectus of higher Vertebrates. It arises 
from the posterior border of the coracoid, and usually spreads to a slight extent over 
the abductor muscles of the fin. For a short distance behind its origin this muscle 
is separated by a low ridge from the axillary portion of the lateral muscle which has 
been described above. 
(3) The Anterior Muscles. 
The anterior muscles which act upon the shoulder girdle are derived from the 
more or less modified forward prolongations of the lateral muscle. 
The deeper fibres of the dorsal moiety and of the latero-ventral portion of the 
ventral moiety are continued forwards beneath the scapular region of the girdle, 
except in the Skates and Rays, and are still recognisable up to their points of origin 
on the chondrocranium ; the superficial fibres in these areas were observed to be 
interrupted by the girdle. As these interrupted fibres behind the girdle serve as 
retractors of that structure, so their anterior homologues serve as protractors. The 
anterior fibres of the mesio-ventral portion are modified to such an extent by the 
development of the visceral arches that they require special consideration. The 
anterior muscles of the pectoral girdle will be dealt with in the following order : — 
(a) Protractor latero-dorsalis pectoralis. 
(b) Protractor latero-ventralis pectoralis. 
(c) Muscles derived from the mesio-ventral portion of the lateral muscles : 
(i) Levator pectoralis. 
(ii) Coraco-arcualis communis. 
(iii) Coraco-branchiales. 
(iv) Coraco-hyoideus. 
(v) Coraco-hyomandibularis. 
(vi) Coraco-mandibularis. 
(a) Protractor latero-dorsalis pectoralis ( p.l.d.p .). — This muscle has, so far as I 
am aware, received no specific designation from previous writers. When present, it is 
derived from the interrupted superficial fibres of the latero-dorsal portion of the lateral 
muscle, which, arising from the anterior border of the scapula, are inserted immediately 
into the main mass of that muscle. It is necessary to make the proviso “ when present,” 
for this muscle is developed in precisely the same degree as its equivalent the r.l.d.p. 
