THE MYOLOGY OF POLYODON 123 
Adductores arcuum branchialium: figure 5, m.adb. (2, 4) 
Within the branchial apparatus there are four pairs of well 
developed adductors. As has frequently been stated, the bran- 
chial cartilages of Polyodon, instead of having the usual some- 
what rounded form, are flattened into very broad thin plates as 
shown in figure 5. From the flat posterior surface of each epi- 
branchial there arises the corresponding adductor muscle. The 
area from which fibers take origin covers the middle portion of 
the cartilage and does not approach the margin at any point. 
The length of the area may be more than a third of the length 
of the whole element. The muscle is covered by a tough apo- 
Fig. 5 A dissection of part of the second gill viewed from behind 
neurotic sheet which binds it to the cartilage and also serves as 
a secondary basis of origin. The somewhat converging fibers 
run obliquely downward and outward and at the ventral margin 
of the epibranchial, cross to the anterior side of the ceratobran- 
chial of the same gill, where they are inserted in a shallow exca- 
vation. Many of the fibers become tendinous toward their end 
and the tendons blend more or less with another very tough 
oponeurosis which covers this part of the muscle. 
Innervation. The rami prae- and post-trematicus, in entering 
the gill above, cross the anterior and posterior faces of the levator 
muscle of the same gill. At its lateral margin they either join 
completely or else anastomose to such an extent that it is no 
