MYODOME AND TRIGEMINO-FACIALIS CHAMBER 313 
The parasphenoid forms the floor of the ventral compartment , 
of the myodome, and whenever the horizontal myodomic mem- 
brane undergoes ossification, the bone so formed forms part of 
the parasphenoid. This bone is thus certainly, in some fishes, 
in part of axial origin, and not simply a dermal bone which has 
gradually sunk inward to its actual position. 
In the Siluridae (Amiurus) there is apparently a much reduced, 
but non-functional, dorsal myodomic compartment, but no 
ventral compartment, that portion of the parasphenoid which 
lies in the prootic region being developed in what corresponds to 
the horizontal myodomic membrane of others of the Teleostei. 
In Amia the myodome corresponds to the dorsal compart- 
ment only of the teleostean myodome, and a strictly similar, 
but non-functional myodomic cavity is found in Lepidosteus 
and Polypterus. The ventral compartment of the teleostean 
myodome is represented, in each of these three fishes, by a canal, 
on either side of the head, which is traversed by the internal 
carotid artery, and which corresponds to the canalis parabasalis 
of Gaupp’s descriptions of higher vertebrates. 
The myodomiec cavity is limited, in the Holostei and Cros- 
sopterygii, to the prootic region, and is there in part subspinal 
and in part prespinal and subpituitary in position. In the non- 
siluroid Teleostei examined, the dorsal compartment of the 
myodome is always more or less prolonged posteriorly into the 
basioccipital region and the ventral compartment frequently 
so prolonged. 
The posterior part of the basioccipital portion of the myodome 
lies between ventrolateral vertebral processes which are quite 
certainly the homologues of the haemal arches of the tail. In 
Hyodon this part of the myodome is an open groove and lodges 
the anterior portion of the median dorsal aorta. In the Cy- 
prinidae part of this groove has become enclosed to form a short 
canal which is traversed by the median dorsal aorta, the enclos- 
ing bone forming the pharyngeal process. 
The conditions in these fishes thus lead inevitably to the 
assumption that the entire dorsal myodomic cavity is a sub- 
vertebral canal similar to the haemal canal in the tail, and that 
