No. 468] MUSCLES OF ACANTHIAS AND RAIA. 915 
tapers to a tendon which is inserted upon the dorsal posterior 
end of the first pharyngo-branchial. It draws the first pha- 
ryngo-branchial forward in the same way that the succeeding 
muscles draw the pharyngo-branchials upon which they are 
inserted. 
The second, third, and fourth medial interarcuales arise from 
the posterior surface of the first, second, and third pharyngo- 
branchials respectively, a little in front of the middle of each 
and are inserted upon the dorsal surface of the second, third, 
and fourth pharyngo-branchials. The point of attachment is 
at about the middle of the cartilage except in the case of the 
second where the attachment is behind the middle. These 
muscles have a larger surface of insertion than of origin and 
decrease in size from in front backwards. 
Lateral System.— A description of the first of the four will 
answer for the first three muscles of this system. The first 
lateral interarcuale has a double origin. The majority of the 
fibers arise from the posterior edge of the anterior end of the 
first pharyngo-branchial and a few from the anterior end of the 
second pharyngo-branchial just in front of the groove (g) for 
the passage of the blood vessel. The fibers are inserted on the 
dorsal surface of the epibranchial along a line which is a con- 
tinuation of the line of insertion of the interbranchial muscle. 
Some of the fibers are inserted so far laterad as to be in front of 
the most dorsal gill rays, so that the muscle appears to be 
almost a portion of the interbranchial which has been forced 
deeper for its origin. The last lateral interarcuale differs from 
the rest only in its origin. Since the fourth and fifth pharyngo- 
branchials are fused the origin is not divided. 
Rata. 
The interarcuales are much reduced in the skate; indeed 
only the lateral system persists while the medial system is rep- 
resented by non-muscular membranes. connecting the pharyngo- 
branchial cartilages in the same way. Each of the muscles of 
the lateral system arises from the posterior edge of the anterior 
end of the pharyngo-branchial and is inserted upon the medial 
end of the epibranchial of the same arch. 
