THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
541 
length of these muscles, which extend fully one-third of the distance between 
the girdle and mandible ; here again the muscles are divided by four myocommata, 
which observation is corroborated by Marion, though Vetter noticed only three. 
Marion’s remark that the muscle in question is attached (in part) to a fascia 
dorsal to the origin of the cera'tohyal” is unintelligible, unless “ceratohyal” is 
a misprint for “ coracohyal” (i.e. the coraco-hyoideus muscle), in which case the 
remark is accurate enough ; the coraco-arcualis is very far removed from the 
hyoid arch. 
A marked deviation is found in the case of Rhina. The coraco-arcuales are of 
enormous strength ; they extend half the distance between the girdle and the mandible, 
and, in the single specimen examined, are traversed by nine myocommata. 
In Raia the coraco-arcuales are very thin straps of muscle which only touch one 
another at their anterior extremities ; they extend not quite half the distance between 
the girdle and the mandible. Their insertions on the coracoid are widely separated 
by the coraco-mandibularis, which in this form is inserted directly on the coracoid 
in the middle line. In no specimen have I been able to identify myocommata. In 
Tiesing’s work the c.a.c. are figured for Torpedo, Rhinobatis, and Raia, though no 
description is given in the text ; no trace of myocommata occurs in these figures. 
(iii) Coraco-branchiales (c.br.). Deepest portion of cervicalis profundus, 
Humphry ; Coraco-branchialis, Wiedersheim, Marion. 
The coraco-branchiales occur with marked constancy in the Selachians as a series 
of muscles which, inserted by separate tendons on the branchial arches, traverse the 
lateral walls of the pericardium to take origin on the -coraco-arcual septum and on 
the coracoid cartilage. In the region of their insertion these muscles form columnar 
masses, but near their origin the distinct muscles become more or less fused with one 
another and become thinner and flatter in appearance. In the shark-like forms the 
C.br. bear a constant relationship to the afferent branchial arteries ; the fused trunk 
of the first and second of these arteries appears immediately in front of the first 
coraco-branchialis muscle (i.e. in the space between it and the coraco-hyoideus), the 
third artery appears between the second and third c.br., the fourth between the third 
and fourth c.br., and the fifth between the fourth and fifth c.br. 
Although Scyllium is perhaps the commonest fish to be dissected in the laboratories 
of this country, its coraco-branchial.musculature, so far as I am aware, has not hitherto 
received any adequate attention. It differs markedly from that of the Piked Dogfish 
(Acanthias), which has been taken in hand by Vetter, and Marion, and evinces 
certain characters which, to my mind, indicate a more primitive grade of evolution. 
I refer to the separation throughout their length of the muscle bundles from each 
branchial arch, and to the fact that all the bundles, with the sole exception of the 
first, are inserted directly on the coracoid bar. 
In Scyllium (fig. 2), then, c.br. is a stout columnar muscle which, arising from the 
