144 Cc. H. DANFORTH 
anterior, anastomose with each other through a longitudinal cord, 
and then continue into the dorsal side of the fin where they 
branch profusely, supplying the muscle and anastomosing with 
one another. Some of the branches pass through the foramina 
in the basalia and between the different basalia to the ventral 
side where a longitudinal trunk is developed from which the 
abductor muscle is supplied. 
Blood supply. The arteries to the fin are several of the splanch- 
nic divisions of the segmental vessel. 
Action. As stated above, the upper muscle is an adductor, 
drawing the fin up against the body. The ventral muscle is its 
opponent, pulling the fin outward and downward. 
xX. CONCLUSION 
The musculature of Polyodon has proved to be rather simple 
in character. How to interpret such simplicity is not always 
evident. On the whole, the resemblance to Acipenser is rather 
close but there are a few points which contrast it very strikingly 
with that form. The system of superficial constrictors, so 
strongly developed in Acipenser, is here reduced to a minimum. 
The condition is even simpler than that found among the elasmo- 
branchs. In this case we are probably justified in regarding 
the simplicity as the result of reduction. Simplification in the 
same direction is also characteristic of higher forms. In the case 
of the protractor hyomandibularis we apparently see another 
advance over Acipenser. With the transverse muscles of the 
pharynx and the pharyngo-claviculares the case is different. To 
be sure, in the possession of these elements Polyodon approaches 
the teleosts more nearly than it does Acipenser, but if one regard 
the muscles as morphological units which phylogenetically can 
only arise from preéxisting muscles, then we are forced to con- 
sider Polyodon more primitive than Acipenser in this particular, 
since the muscles in question have apparently been lost in the 
latter form. The question of the ventral fin is also of interest 
in this connection. The numerous radialia are interpreted by 
von Davidoff as the result of division of an element which in 
most forms is a single plate and on this ground, chiefly, he would 
