THE MYOLOGY OF POLYODON 119 
MM. retractor hyomandibularis et opercularis: figure 2, m.ret. 
The retractor hyomandibularis and the opercularis are prac- 
tically identical with similar muscles in Acipenser (Vetter ’78). 
In Polyodon, however, they are confluent at their contiguous 
margins. Nevertheless the line of union is probably indicated 
by differences in size of the muscle bundles, those of the retractor 
portion being much the larger. The combined muscle arises from 
a rather large groove on the side of the cranium, extending from 
near the articulation of the hyo-opercular back about to the 
level of the dorsal corner of the opercular cleft (fig. 4, m.ret.). 
Some of the posterior fibers may arise cutaneously from the 
dorsal margin of the opercular flap. Few, if any, arise from the 
overlying frontal bone as they do in Acipenser. From its origin 
the muscle spreads out fan-like and descends to its insertion on 
the hyomandibular, to which it is attached from the medial 
articulation to the distal end. Beyond the end of the hyoman- 
dibular the fibers, probably all belonging to the opercularis proper, 
are inserted along the upper edge of the operculum, or more 
strictly, in the skin immediately beneath it. The most posterior 
fibers reach about as far caudad as the end of the dorsal spicule 
of the opercular bone. These fibers have a relation to the oper- 
culum which is identical with that which the posterior geniohyoid 
fibers bear to the similar branchiostegal ray below. They prob- 
ably represent the two ends of the same primitive superficial 
constrictor. 
Innervation. They are supplied by the hyomandibular branch 
of the facial nerve which runs along the cartilage and sends 
superficial branches over the muscles. The ramus oticus tri- 
gemini passes through the occipital cartilage and runs over the 
surface of the muscle. It sends twigs to the muscle and also 
passes through it to anastomose with a branch of the vagus. 
Whether either of these nerves supply motor fibers to the muscles 
cannot be stated. 
Blood supply. ‘The blood supply is through the hyo-opercular 
artery and the accompanying vein. There are also twigs from 
the trunk of the second efferent branchial artery. 
