WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEURONECTES. 21 
median region of the ethmoid, thus bridging over the left ethmoid notch 
and leaving between the mes-ethmoid and the region of the anterior end 
of the right supraorbital cartilage an orifice (for. olf. s.), which corre- 
sponds to the notch on the right. In other specimens I find that both 
wings of the ethmoid have sent out processes to fuse with the mes- 
ethmoid, thus converting both notches into foramina for the passage of 
the olfactory nerves to their capsules on the front of the ethmoid. 
In this model a bent wire is inserted into the mes-ethmoid in the median 
plane to aid in locating the position of that plane, — the plane in which 
the future interorbital septum is to develop. There is as yet no trace 
of this septum in the specimen modelled ; but Figure 18 (Plate 4) shows 
a cross section of the head of a fish (P. americanus) of this stage, which 
does indicate the position of the future interorbital septum. The fine 
vertical lines outside the figure represent the projection of the sagittal 
plane of the fish, A small bar of cartilage (arc. eth. m.) is seen in cross 
section above the mes-ethmoid. ‘Traced anteriorly a few sections, this 
fuses with the ethmoid. Traced posteriorly it soon unites with the 
thin fused trabecule cranii not far from where they pass over into the 
ethmoid. It is, then, a slanting bar, or arch, from near the anterior 
end of the trabecule cranii to the posterior face of the ethmoid. In an- 
other specimen (Figure C’, p. 24) this arch has become larger and ap- 
pears as the forward prolongation of the trabecule (¢rb.). In the space 
beneath this arch lie the oblique eye muscles, two of which (the right 
and left inferior oblique) appear in Figure 18. The same figure shows 
that the migrating eye may exert pressure directly on the cartilage, for 
the left eye-ball is indented by the left wing of the ethmoid. 
In another specimen of this stage, which had lost the migrating eye 
in the process of turning, there were certain peculiarities worthy of con- 
sideration. This fish, too, had a well-developed median arched cartilage 
on the posterior face of the ethmoid. The right superior oblique muscle 
had its origin at the angle produced by the junction of the arch and the 
body of the ethmoid. The inferior oblique was attached lower, at the 
angle made by the union of the ethmoid and the trabecule. The pos- 
terior face of the ethmoid is the usual place of attachment for these 
muscles, though a specimen of B. maculatus had both the inferior and 
Superior oblique muscles attached on the median arched bar. The most 
noticeable peculiarity of this specimen was shown in the origin of the 
supraorbitals. As I have said, there was no eye present on the left 
side. The anterior end of the left supraorbital bar still persisted in this 
specimen in the form of a stub projecting backward and slightly upward 
