110 Cc. H. DANFORTH 
Blood supply. The ophthalmic branch of the external carotid 
supplies the rectus muscles and more anterior branches of the 
same artery supply the two obliques. The veins of the orbit 
are tributaries of the jugular. 
Rather an unexpected tendency to variation was met with in 
connection with these muscles. The internal rectus, referred to 
above as the smallest of the group, was twice—once in a medium 
sized individual, once in a small one—found to be double through- 
out most of its extent. Both parts were tendinous near their 
insertion. In a third specimen, a large fish, careful dissection 
failed to reveal any trace of it on the left side although present 
and normal on the right. Such variability may be indicative of 
a retrograde tendency on the part of this element. An intensive 
study of the eye muscles of a large number of specimens might 
yield interesting results. 
III. MUSCLES OF THE MANDIBULAR AND HYOID ARCHES 
M. geniohyoideus: figures 1 and 2, m.gha., m.ghb. 
In Polyodon the primitive superficial muscles of the head are 
represented ventrally by a thin lamina, the geniohyoideus, cross- 
ing the space bounded laterally on either side by the ramus of 
the mandible and the branchiostegal ray and posteriorly by the 
margin of the opercular flap (figs. 1 and 2). As indicated in the 
figures, the muscle does not extend forward quite to the symphy- 
sis of the jaw. The fibers arise laterally and with the exception 
of the most posterior, are all inserted in a median aponeurotic 
thickening. As is frequently the case, this muscle is in two parts, 
anterior and posterior. The anterior part arises (a) from Meckel’s 
cartilage, beginning at a point a little behind the symphysis and 
extending back nearly to its posterior end, and (b) from the 
overlying dentary bone of the same region. In a 74 mm. speci- 
men, apparently all the fibers arise directly from the cartilage. 
In reaching their insertion the anterior fiber bundles run some- 
what obliquely backward and inward, the intermediate ones are 
transverse, and the posterior run obliquely forward and inward 
as indicated in figure 1. These last are very nearly parallel 
