No. 468.] MUSCLES OF ACANTHIAS AND RAIA. 923 
Rata. 
In Raia this muscle (Fig. 15, Có») which forms the lateral 
wallof the pericardial cavity, arises from the pectoral girdle. 
Farther forward the muscle expands and the tendon gives place 
to muscle fibers which are inserted on the anterior process of 
the basibranchial and the membranous floor of the mouth. The 
other fibers run dorsally and become attached to the ends of the 
cerato- and hypobranchials. The divisions between these parts, 
as in Acanthias, permit the passage of blood vessels, and extend 
nearly back to the origin dividing the muscles into superimposed 
layers. 
Coraco-arcuales communes. 
Acanthias. 
The paired coraco-arcuales communes muscles (Figs. 4, 5, 
Cac) lie immediately beneath the skin and arise from the cora- 
coid region of the girdle. The fibers run inward and forward, 
and the medial fibers become inserted in the strong membrane 
which forms the floor of the pericardium, while the lateral parts 
are inserted upon the fascia dorsal to the origin of the cerato- 
hyal. Each muscle is crossed by four myosepta, which makes it 
resemble somewhat the ventral body muscles behind the girdle. 
Raia. 
In the skate (Figs. 3, 15, Cac) this muscle closely resembles 
that in Acanthias in origin, direction, and insertion. 
Coraco-hyomandibularis. 
This wide, comparatively thick, and long muscle occurs only 
in Raia (Figs. 3, 15, Chm) of the forms studied, where it is the 
largest and most important of the ventral longitudinal muscles. 
It arises from a fascia in the middle line, and runs forward nearly 
to the division of the ventral aorta. The muscle runs obliquely 
