THE MYOLOGY OF POLYODON 111 
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with, and not clearly separated from, the fibers of the posterior 
part of the muscle which they slightly overlap. The posterior 
part of the geniohyoid takes its origin (a) from the ceratohyal 
between the groove for the facial nerve and the posterior angle 
of the cartilage, (b) from the ventral margin of the interhyal, 
and (c) from a line along the ventral margin of the branchio- 
stegal ray. The fibers do not. arise directly from the ray, but 
especially in younger individuals, from the skin beneath (medial 
to) it. The anterior muscle bundles are directed obliquely for- 
ward to the median line, the posterior become more and more 
transverse in their direction. Some of the latter seemingly cross 
to the opposite side without any median insertion. 
Innervation. The anterior part is supplied by the end twigs 
of the ramus maxillaris inferior trigemini, and the posterior by 
several branches of the ramus hyoideus facialis. In addition to 
these nerves the ramus mandibularis facialis externus courses over 
the surface of the muscle immediately superficial to the main 
trigeminal branch. In Amia these two nerves unite (McMurrich 
’85) but in Polyodon I could not detect any anastomosis between 
them, and apparently the former is distributed entirely to the 
lateral line organs as suggested by van Wijhe (’84). 
_ Blood supply. Terminal branches of the facial artery in front 
and of the hyo-opercular behind ramify over the ventral surface 
of the muscle. The corresponding veins lead away from it. The 
anterior part also receives a dorsal supply through small arteries 
originating in anastomoses between the end of the facial artery 
and descending branches of the lateral hypobranchial. The pos- 
terior part may get a little of this supply. The veins of the 
dorsal side drain into the inferior jugular. 
Action. Contraction of the muscle tenses the pouch-like fold 
beneath the jaw and between the opercular flaps. Action of this 
muscle must tend to prevent spreading of the rami of the mandi- 
ble and of the hyoid and also assist in drawing the opercular 
fold against the body, thus performing an accessory function in 
inspiration and deglutition. 
As to the homologies of this muscle, it seems to me that it is 
to be regarded simply as geniohyoid. The anterior part might 
