MYODOME AND TRIGEMINO-FACIALIS CHAMBER 225 
The myodome of my 51-mm. specimen of Hyodon thus lies 
in part beneath the pituitary opening of the brain case and in 
part posterior to that opening, a part of it thus being prechor- 
dal and the remainder chordal in position, or, in the terminology 
employed by Froriep (’02 a), the one prespinal and the other 
spinal. The spinal portion is formed, throughout part of its 
length, by two distinctly different parts, one dorsal and the other 
ventral, the two being completely separated from each other 
in part by cartilage and in part by membrane which forms a 
direct anterior prolongation of the cartilage. In the prespinal 
portion these two compartments of the spinal portion are 
confluent because of the breaking down of the separating wall 
(the horizontal myodomic membrane) by the structures that 
here enter or leave the dorsal compartment. The canal tra- 
versed by the internal carotid arteries as they run upward in the 
median vertical myodomic membrane lies in the level, anteriorly 
prolonged, of the dorsal myodomic compartment, but it forms 
no part of either compartment of the myodome. 
The dorsal compartment of the myodome of Hyodon is lim- 
ited to the subpituitary and postpituitary regions, and although 
both of these parts le in the prootic region, the postpituitary por- 
tion, which lies beneath the prootic bridge, may be alone re- 
ferred to as the prootic portion of the compartment. The ventral 
compartment has prootic, subpituitary, and prepituitary por- 
tions. The dorsal compartment is directly continuous posteri- 
orly with the anterior end of the aortal groove, which extends 
the full length of the basioccipital region. The ventral com- 
partment is not continuous with the groove and it does not 
extend posteriorly as far as the dorsal compartment. It lies 
between the floor of that compartment and the parasphenoid, 
lodges the hind ends of the recti interni, and is traversed by the 
internal carotid arteries, the palatine branches of the faciales, 
and the efferent pseudobranchial arteries. The dorsal com- 
partment lodges the pituitary veins and the musculi recti ex- 
terni, these muscles entering it from the orbits and leaving it by 
its posterior opening. The nervus abducens of either side perfo- 
rates the roof of this compartment to reach and supply the rectus 
