MYODOME AND TRIGEMINO-FACIALIS CHAMBER 241 
passing through this region the hind ends of the parasphenoid 
are cut, one lying on either side of a median ridge on the ventral 
surface of the basioccipital, the median dorsal aorta lying ven- 
tral to the ridge (fig. 27). Here, unfortunately, one or more 
sections are missing in my series. In the next anterior existing 
section (fig. 26) there is a circular space in the basioccipital, 
in exactly the position of the notochordal space in the next pos- 
terior section of the series, but somewhat larger than it, and this 
space lodges the hind ends of the recti externi. ‘The aorta here 
begins to separate into a lateral dorsal aorta on either side. 
Proceeding anteriorly in the sections, the myodomic cavity 
increases in size, and that part of the basioccipital in which it 
lies forms a large median, dorsally projecting, and rounded 
ridge. The parasphenoid now extends across the median line 
fig. 25). Still farther forward the fenestra ventralis myodomus 
begins, the parasphenoid closing it ventrally and having a 
broad but low median ridge which projects upward into the 
fenestra. This median ridge then sends upward a longitudinal 
ridge on either side, and in the space between these two ridges 
the recti interni make their appearance, separated from the recti 
externi by loose connective tissue which does not form a definite 
membrane (fig. 24). 
Farther forward, the two lateral ridges on the dorsal surface 
of the parasphenoid vanish, leaving a flat median ridge, and 
the hind end of the hypophysis is there cut in the sections. This 
latter organ is large, lies wholly in the myodome, and projects 
posteriorly ventral to the roof of the myodome, here formed by 
the horizontal processes of the prootics. Anterior to the an- 
terior edges of these latter processes the hypophysis is connected 
with the brain by a small stalk of nervous material, which per- 
forates the membrane which there forms the roof of the myo- 
dome and the floor of the cavum cerebrale cranii. 
The myodome of this fish is thus, up to this point, strictly 
normal, except that the hind end of its dorsal compartment 
is enclosed in the basioccipital, and that the recti externi appa- 
rently have their origins on the anterior end of the notochord 
instead of ventral to it. 
