MYODOME AND TRIGEMINO-FACIALIS CHAMBER 235 
A second membrane, evident also in the preceding figure, here 
closes the fenestra ventralis myodomus, the parasphenoid lying 
directly against its ventral surface. The median vertical mem- 
brane still extends between these two membranes, separating 
the myodome into lateral halves. 
Still farther forward in the sections, in the region of the hind 
end of the hypophysis (fig. 15), the median portion of the mem- 
brane that forms the roof of the myodome, and hence also the 
median portion of the floor of the cavum cerebrale cranii, grad- 
ually descends, between the recti externi, on to the membrane 
forming the floor of the myodome, the two membranes fusing 
with each other there, and so forming a thick membrane which 
is both the median portion of the floor of the cavum cerebrale 
cranii and the median portion of the membrane closing the 
fenestra ventralis myodomus, the parasphenoid lying directly 
upon its ventral surface. The myodomic cavity, which here still 
belongs only to the dorsal compartment of the myodome, is 
thus separated into lateral halves, the hypophysis projecting 
ventrally between the two halves of the compartment, and 
each half lodging the related musculus rectus externus. 
Still farther forward in the sections (figs. 13 and 14), the hind 
ends of the recti interni appear between the parasphenoid and 
the membrane closing the fenestra ventralis myodomus, that 
membrane thus being the horizontal myodomic membrane, 
and the space beneath it the ventral compartment of the my- 
odome. The membrane forming, on either side, the roof of 
the related lateral half of the dorsal compartment of the myo- 
dome, now has its mesial attachment on the dorsal surface of 
the horizontal myodomic membrane, the latter membrane thus, 
in its lateral portions, separating the two compartments of 
the myodome, while its median portion forms part of the roof 
of the ventral compartment of the myodome and part of the 
floor of the cavum cerebrale cranii. The conditions here are 
accordingly similar to those in the 63-mm. specimen of Trigla 
hirundo. 
Still farther forward, that lateral part of the horizontal mem- 
brane that, on either side, separates the two compartments 
