THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
547 
II. Holocephali. 
(Chimera monstrosa, Callorhynchus antarcticus.) 
1. Shoulder Girdle. 
(l) The Lateral Muscle. 
The lateral muscle conforms, both in structure and external appearance, to the 
typical condition in Elasmobranch fishes. This condition has already received 
sufficient notice in the section on Selachii. 
(2) The Posterior Muscles. 
The parts of the lateral muscle which go to form the retractors of the pectoral 
girdle in the Holocephali have attained a much higher grade of specialisation than 
has been encountered among the Selachians. The retractor muscles under considera- 
tion are not merely interrupted portions of the segmented lateral muscle, but 
separate sheets, enclosed in their own fasciae, and devoid of all trace of myocommata 
(except in the two lower sections of the mesio-ventral portion). They will be taken 
in the following order : — 
(a) Retractor dorsalis pectoralis. 
(b) Retractor later o-ventralis pectoralis : 
(i) Externus. 
(ii) Internus. 
(c) Retractor mesio-ventralis pectoralis : 
(i) Superior. 
(ii) Medius. 
(iii) Inferior. 
(a) Retractor dorsalis pectoralis ( r.d.p .). — This is a broad thin sheet of muscle 
which, arising from the portion of the lateral muscle immediately above the lateral 
line, runs in a direct, antero-posterior direction to be inserted on the inner aspect of 
the posterior rim of the scapula and of the backwardly projecting distal horn of that 
cartilage. In Chimeera the muscle is of considerable length, extending to the third 
myocomma behind the scapular horn, from which myocomma the majority of its 
fibres take origin. In Callorhynchus, on the other hand, the muscle is relatively 
short, its fibres taking origin from the myocomma which touches the tip of the 
scapular horn. In both genera the scapular horn extends above the level of the 
latero-dorsal portion of the lateral muscle ; the retractor muscle, therefore, comprises 
part of the mesio-dorsal portion in addition to the entire latero-dorsal portion ; it 
thus corresponds to the r.m.d.p. and r.l.d.p. of Selachians. As, however, it is not 
divided longitudinally, it is designated here simply as the r.d.p., or the retractor 
muscle which is derived from the dorsal moiety of the lateral muscle. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 21). 
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