130 Cc. H. DANFORTH 
aorta and at about the level of the second gill. The single 
median muscle that results turns downward and backward in 
the opercular fold (fig. 3). In this region it is largely tendinous. 
After coursing posteriorly for a few millimeters it again turns 
forward, thus giving the muscle when at rest a Z-form. At the 
point where it turns forward it is joined by a few fibers (m.bmd.’) 
from the opercular folds. Anteriorly this median muscle again 
divides into lateral halves which are inserted on the rami of the 
mandible very near to the median line. 
Innervation. Repeated dissections failed to disclose the nerve 
of supply. In those fish where it is described (e.g., Amia, Allis) 
it is a branch of the first spinal nerve. 
Blood supply. Arterial blood is supplied by the hypobranchial 
arteries and the terminal twigs of the facial with which they 
anastomose anteriorly. The veins are tributaries of the inferior 
jugular. 
Action. No amount of stretching suffices to pull the muscle 
into a straight line, so the anterior and posterior parts doubtless 
act separately, both serving as depressors. Their action, how- 
ever, must be feeble, for the muscle is very small. 
MeMurrich (l.c.) calls attention to the fact that this muscle 
diminishes as we ascend the scale and points to Amia as probably 
the last piscine form to show it. In that fish it is in relation 
with the second, instead of the third arch, as in Polyodon and 
also Acipenser. The few fibers which run into the opercular 
fold and presumably tense the median fascia into which the 
geniohyoid is inserted may be the last remnant of a primitive 
connection of this muscle with the system of superficial con- 
strictors. 
M. coraco-arcualis: figures 3 and 8, m.coar. 
This is the hyoclavicularis or sterno-hyoid muscle. It takes 
origin chiefly from the anterior third of the cartilaginous part 
of the pectoral arch (fig. 8, m.coar.). The fibers arise directly 
from a long crescentic area located medially and toward the 
ventral side of the cartilage. A conspicuous portion of the mus- 
