MYODOME AND TRIGEMINO-FACIALIS CHAMBER 215 
certainly formed by aortal supports, the remainder of each 
primitive ventrolateral vertebral process being represented in 
the little cartilaginous ridge that gives attachment to the 
ligament which runs outward, rib-like, in the related intermus- 
cular septum. 
Returning now to the descriptions of Hyodon and proceeding 
forward in the sections, figure 10 shows that a ridge of bone grad- 
ually appears lateral to the tall ridge that bounds on either side 
the aortal groove, the appearance in these sections somewhat 
suggesting, excepting in the absence of cartilage, the conditions 
shown in Hay’s figures 5 and 6 of trunk vertebrae of Amia, 
where the aortal supports are attached to the ventromesial sur- 
faces of the lower arches (parapophyses, Hay). Still farther 
forward, in sections through the hind ends of the parasphenoid 
(fig. 9), the mesial one of these two processes has disappeared 
while the lateral one persists as a stout low process, this giving 
a broad ventral edge to the aortal groove. The lateral walls of 
the aortal groove are now formed by the entire ventrolateral 
processes, and not simply by the aortal supports, and the bony 
deposits on either side that fill the space between these proc- 
esses and the dorsolateral vertebral processes (upper arches) 
has been excavated to form the recessus sacculi. Hence the 
basioccipital is here W-shaped in transverse section, the two 
grooves on the dorsal surface of this W each forming the ven- 
tral portion of the related recessus sacculi, and the grooves of 
opposite sides being separated from each other by a tall median 
plate formed by part of the basioccipital. The notochord lies in 
the ventral end of this median plate, dorsal to the bottom of the 
aortal groove, but is here represented simply by a notochordal 
space. The exoccipital of either side has vertical and horizon- 
tal plates, the former forming the dorsal portion of the side wall 
of the related recessus sacculi and the mesial portion of the other 
the roof of the recessus, the mesial edge of the latter plate rest- 
ing upon the dorsal end of the tall median plate of the basioc- 
cipital and forming, with its fellow of the opposite side, the 
floor of the cavum cerebrale cranii. The median groove on 
the ventral surface of the W here lodges the hind ends of the 
