540 
CAPTAIN E. W. SHANN ON 
The above description holds good for all the shark-like types. In Raia the l.p. 
is also present, though considerably reduced in extent. 
The origin of the levator pectoralis, whether in the Sharks or Rays, is continuous 
anteriorly with that of a small muscle which runs to the last branchial arch ; these 
two muscles constitute the trapezius of earlier writers, which from its superficial 
position and double function has received abundant notice. 
(ii) Coraco - arcualis communis (c.a.c.). Cervicalis profundus pars, 
Humphry ; Coraco-arcualis communis, Marion. 
On the ventral aspect of Elasmobranch fishes, anteriorly to the pectoral girdle, the 
lateral muscle is broken up into a complex series of separate muscles coming from 
the branchial, hyoid, and mandibular arches to be inserted on the coracoid portion of 
the girdle. These muscles, however, anastomose to a greater or less extent before 
reaching the girdle; so that a pair of muscle masses, derived from the separate 
portions of each side, occur immediately in front of that structure on either side 
of the. mid -ventral line ; each of these is named coraco-arcualis communis. The 
anastomosis spoken of above is not gradual, but takes place at a well-defined junction, 
namely, at a strong fascia — the coraco-arcual septum. The coraco-arcuales communes 
form internally the floor of the pericardium. 
In Scyllium the c.a.c. is intercepted by four transverse myocommata, which 
enhances its resemblance to the portion of the lateral muscle immediately behind the 
coracoid — the retractor mesio-ventralis pectoralis inferior. Humphry regarded this 
muscle, which he designated, together with the coraco-hyoideus (to be described 
below), cervicalis profundus, as the anterior continuation of the “pectoralis” ( = r.m.v.p. 
inf.); moreover, he adds that the cervicalis profundus “presents transverse septa 
corresponding in number and position with the branchial cartilages.” The former 
surmise is highly probable, since Humphry observes that this muscle “ extends with 
the septum for some little distance upon the ventral aspect of the fin,” and a similar 
condition has been noted above in treating of the insertion of the r.m.v.p.inf. ; in 
fact, the lateral range of these two muscles on either side of the median line at the 
level of their insertions on the girdle is almost identical. With regard to the trans- 
verse septa recorded by Humphry for Mustelus, it is to be supposed that the most 
anterior of these is the coraco-arcual septum, while the remaining four are equivalent 
to the four myocommata which have been noted in Scyllium ; they thus correspond in 
number to the branchial arches, but, in the latter type, they cannot be described as 
corresponding in position. The coraco-arcual septum in Scyllium is wedge-shaped, 
the thin edge of the wedge extending in a horizontal plane between the coraco- 
mandibularis and coraco-hyoideus on the ventral aspect, and the first coraco-branchialis 
on the dorsal aspect. 
In Galeus the coraco-arcuales are exceedingly short, and are not traversed by 
distinct myocommata. Acanthias differs from the foregoing types in the considerable 
