No. 468.] MUSCLES OF ACANTHIAS AND RAIA. 907 
from the tendinous fasciz connecting these cartilages, while the 
fibers on the median side arise from the fascia between the longi- 
tudinal muscles (Fig. 5 shows an instance of origin at the end 
of the common coraco-arcualis). This point is of some impor- 
tance in the interpretation of the condition in Raia. The fibers 
near the middle line are inserted upon the anterior surface of 
the cerato-branchial cartilage, while the lateral fibers are contin- 
uous with those of the dorsal side as noted above. Often the 
muscle fibers stop at a gill ray, the tissue becoming aponeurotic, 
and then the muscle fibers continue on the other side. 
These interbranchial muscles draw the gill radii together and 
thus aid in the contraction of the gill basket. In the case of 
the hyoid arch, the second dorsal and ventral constrictors replace 
the interbranchialis, since here the gill radii lie immediately 
beneath the surface (constrictor) muscles. 
Rata. 
The interbranchiales of Raia are much like those of Acanthias 
in position and number, the muscle of the first arch being 
replaced by the posterior part of Csv2 and Csd2. Owing to the 
shape of the branchial region they do not form a continuous arc 
but are divided into a dorsal and a ventral portion, so that the 
line of division coincides with the angle separating the halves of 
each branchial arch. The muscle fibers of the dorsal portion 
arise from the aponeuroses between the dorsal constrictors, and 
run ventrally (Fig. 12) until they become inserted upon the 
dorsal surface of the epibranchial cartilages or upon the ray 
which runs out from the angle to the horizontal tendon. The 
ventral fibers take their origin from the tendons between the 
ventral constrictors and run dorsally to their insertion upon the 
ventral surfaces of the cerato-branchials or upon the horizontal 
ray just referred to. The fibers never become continuous, as in 
Acanthias, from dorsal to ventral surfaces. A few bundles of the 
most median fibers of the ventral portion are overdeveloped, and 
have extended so as to take their origin from fascize covering the 
coraco-mandibularis muscle (Fig. 3, /nzor). A similar condition 
was noted above for Acanthias but these fibers were not as 
