THE SHOULDER GIRDLE AND PECTORAL FIN OF FISHES. 
543 
on the outer ventral aspect of the basibranchial plate. It shows no sign of division 
into segments such as we have observed in Scyllium. 
Turning now to Rhina, we again approach new ground, but, with a few trivial 
exceptions, the characters of the coraco-branchial musculature are readily conformable 
to type. In accordance with the flattened shape of their possessor, these muscles 
are more flattened than in the shark-like forms. The arteries pass outwards in the 
normal manner. 
C.br. 1, inserted mainly on the first hypobranchial and in part also on the dorsal 
aspect of the basihyal, runs backwards almost in the same horizontal plane to its 
origin on the anterior part of the dorsal aspect of the coraco-arcual septum. 
The remaining coraco-branchiales, inserted in turn into the succeeding branchial 
arches, take origin from the coracoid itself beneath the coraco-arcuales communes. 
C.br. 2-4 become inextricably fused with one another for a short distance in front 
of their origins ; they then separate to go to their insertions on the appropriate arches. 
These insertions are, as usual, mainly on the hypobranchials, but there is an increasing 
tendency to send fibres to the ceratobranchials also as we proceed from before back- 
wards. The hindermost muscle of this series, c.br. 5, is composed-of a very broad 
sheet which is distinctly divided into two portions. 
In Raia we meet with a very specialised condition of the coraco-branchial muscular 
system. From its attachment to the angle of pectoral girdle, immediately lateral to 
the origin of the c.a.c. and contiguous dorsally with the l.p., the muscle runs obliquely 
inwards, keeping its horizontal plane and showing no signs of dividing into separate 
bundles. It passes external to the trunk which divides into the third, fourth, and 
fifth afferent branchial arteries, to its main area of insertion on the anterior process 
of the basihyal and the membranous floor of the mouth. In the dorso-lateral region 
it also gives off a few fibres which are attached to the hypo- and ceratobranchials ; 
these are the only vestiges of the c.br. 2-4. Posteriorly there is a strong connection 
with the fifth ceratobranchial ; this represents c.br. 5. I have been unable to detect 
a trace of c.br. 1. 
(iv) Coraco-hyoideus (c.hy.). Cervicalis profundus pars, Humphry ; Coraco- 
hyoideus, Vetter, Wiedersheim, MIrion. 
The coraco-hyoideus is usually a massive muscle which extends from the coraco- 
arcual septum to the basihyal cartilage ; its length varies inversely with that of the 
coraco-arcualis communis. The c.hy. is not as a rule marked by myocommata, though 
Vetter figures one such division in Acanthias and Heptanchus. 
In Scyllium, Galeus, and Acanthias the c.hy. arises from the ventral face of the 
coraco-arcual septum. Externally, only its lateral surface can be seen, the median 
portion being covered by the coraco-mandibularis, to be described later. It runs 
forwards, in contact with its fellow of the opposite side, and finds insertion on the 
posterior and postero-dorsal border of the basihyal. Its relative length is greatest in 
