THE MYOLOGY OF POLYODON 143 
plexus of the fifth and sixth spinal nerves. It follows the basal 
cartilage and sends twigs up into the muscle. 
Blood supply. The muscle is supplied by ventral branches of 
the subclavian artery. 
Action. This muscle pulls the fin downward and inward. It 
also tends to rotate the anterior part of the fin and in this func- 
tion the fibers inserted on the first ray are of especial importance. 
IX. MUSCLES OF THE PELVIC FIN 
The pelvic fin is the most complicated in structure of all the 
fins. Von Davidoff (’79) in his paper on the hind limbs of fishes 
includes Polyodon among the ganoids studied. He gives figures 
of the skeleton of the ventral fin and a brief description of the 
musculature. The innervation was not fully worked out for 
Polyodon. So far as the present work goes it is in accord with 
the results obtained by him. 
Following the analogy of the pectoral fin, the muscles here 
might also be classed as a dorsal adductor and a ventral abductor. 
The dorsal musculature is in two parts, superficial and deep, like 
the musculature of a median fin. The two parts are separated 
'by an incomplete tendinous septum. The fibers of this part 
arise from the aponeurotic covering of the lateral musculature 
and are inserted beneath the rays of the fin. The deep part is 
peculiar in that its most superficial fibers seem to be in direct 
continuity with the lateral body musculature. The deeper fibers 
arise (a) from fhe upper surface of the basalia, (b) from the 
dorsal lateral margin of the crest of the same, and (c) from the 
radialia. The muscle is divided by the crests of the basalia and 
by thin connective tissue sheaths into as many subdivisions as 
there are radialia, thirteen or sometimes fourteen. 
The ventral muscle, which is an abductor in function, is rela- 
tively simple. It arises (a) from the median ventral aponeurosis, 
(b) from the basalia, and (c) from the radialia. It is inserted 
on the ventral side of the fin in the same manner as is the adduc- 
tor muscle on the dorsal side. 
Innervation. The branches of several spinal nerves, apparently 
four or five, of which number eighteen is probably the most 
