MYODOME AND TRIGEMINO-FACIALIS CHAMBER 229 
The above statements all referred only to the adult of this fish, 
for I at that time had no specimens small enough to be sectioned. 
I have, however, since prepared a series of transverse sections 
of a 65-mm. specimen, in which [ now find the vertical mem- 
brane above referred to descending from the ventral surface of 
the membranous pituitary sac, as in Hyodon. That part of it 
which, in the adult, lies beneath the horizontal membrane and 
extends to the hind end of the myodome is, in this 65-mm. spec- 
imen, simply a delicate line of connective tissue, but it would 
nevertheless seem to represent a remnant of a wall which pri- 
'marily separated this part of the myodome into two parts, one 
on either side, as will be later explained. The horizontal mem- 
brane is practically as I described it in the adult. In its ante- 
rior portion it is not strongly developed, and there arises, on 
either side, from a layer of tissue which lines the internal surface 
of the side wall of the myodome cavity and is continued outward 
around the ventral end of the wall and then upward a certain 
distance along its external surface. The parasphenoid rests, on 
either side, upon the ventral surface of this tissue, and a longi- 
tudinal ridge on either side of the dorsal surface of the bone 
projects upward into that part of the tissue which lines the in- 
ternal surface of the side wall of the cavity; this part of the par- 
asphenoid certainly being an ossification developed in relation 
to the tissue. Along the line of origin of the horizontal mem- 
brane, the cartilage of the side wall of the myodome is slightly 
constricted and imperfect, suggesting a segmentation line similar 
to that shown by Schauinsland (’05) where a rib is in process 
of being segmented from a lower arch in the vertebral region 
of certain other fishes. Near the hind end of the myodome, 
beginning slightly anterior to the point where the recti interni 
terminate, that part of the cartilage of each lateral wall of the 
myodome that lies ventral to this segmentation line gradually 
passes, without any line of demarcation, into dense fibrous tis- 
sue which forms the ventral end of each lateral wall of the my- 
odome. The parasphenoid here lies against the ventral surface 
of this tissue, and the longitudinal ridge on either side of the 
dorsal surface of the bone extends upward along the mesial sur- 
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