242 
ZOOLOGY: E. P. ALUS 
compartment lodges the hind ends of the musculi recti externi, and is always 
traversed by a cross-commissural venous vessel formed by the pituitary veins. 
The ventral compartment lodges the hind ends of the musculi recti interni, 
and is traversed by the internal carotid and efferent pseudobranchial arte ies 
and the palatine branches of the nervi faciales. 
The parasphenoid forms the floor of the ventral compartment of the myo- 
dome, and whenever the horizontal myodomic membrane undergoes ossifica- 
tion, the bone so formed forms part of the parasphenoid. This bone is thus 
certainly, in certain fishes, in part of axial origin, and not simply a dermal 
bone that has gradually sunk inward to its actual position. 
In the Siluridae (Ameiurus) there is apparently a much reduced, but non- 
functional, dorsal myodomic compartment, but no ventral compartment, 
that portion of the parasphenoid that 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 compartment only of the 
teleostean myodome, and a strictly similar, but non-functional, myodomic 
cavity is found in Lepidosteus and Polyp terus. The vejntral compartment of 
the teleostean myodome is represented, in each of these three fishes, by a canal, 
on either side of the head, that is traversed by the internal carotid artery, and 
that corresponds to the qanalis parabasalis of Gaupp's 1 descriptions of higher 
vertebrates. 
The myodomic cavity is limited, in the Holostei and Crossopterygii, to the 
prootic region, and is there in part subspinal and in part prespinal and sub- 
pituitary in position. In the non-siluroid Teleostei examined, the dorsal 
compartment of the myodome is always more or less pro'onged posteriorly 
into the basioccipital region, and the ventral compartment is frequently so 
prolonged. 
The posterior part of the basioccipital portion of the myodome lies between 
ventro-lateral vertebral processes that 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 Cyprinidae,part of this groove has become enclosed to form a short canal, 
which is traversed by the median dorsal aorta, the enclosing bone forming the 
pharyngeal process. 
The conditions in these fishes thus leads inevitably to the assumption that 
the entire dorsal myodomic cavity is a subvertebral canal similar to the haemal 
canal in the tail, and that the dorsal aorta has been excluded from it because of 
the formation of a circulus cephalicus. What the primary relations of the 
hypophysis and pituitary veins to this preexisting canal were, is problematical, 
but they became lodged in its anterior portion and so gave rise to the condi- 
tions actually found in Lepidosteus and Polypterus. The musculi recti ex- 
terni then secondarily invaded this space by traversing the foramina for the 
