MYODOME AND TRIGEMINO-FACIALIS CHAMBER 267 
tain of the Teleostei (Salmo, Gasterosteus), and skeletogenous 
tissues capable of taking a membranous form would certainly 
be left in its place. The membranous tissues that would then 
represent the presphenoid bolster would not offer a firm point 
of attachment for the rectus muscles, and it would be wholly 
natural for certain of them to seek more solid points of origin, 
and one of them actually has, in most of the Teleostei, acquired 
such an origin by first creeping downward on to the dorsal sur- 
face of the parasphenoid and then pushing posteriorly in the 
open end of the persisting remnant of the palatine canal of my 
descriptions. This muscle actually is the rectus internus, but 
it is possible that it was primarily the rectus inferior, that muscle 
and the rectus internus undergoing an exchange of function 
and so giving rise to that manner of innervation of these muscles 
that I have described in several of these fishes (Allis, ’03, ’09), 
and which I now find to be apparently definitely related to 
the presence of a functional ventral myodomic compartment. 
Where that compartment is wanting, as in Amiurus, or present 
but non-functional, as in Lepidosteus, Polypterus, Polyodon, 
Acipenser, and higher vertebrates, these muscles are innervated 
approximately as they are in Amia (Allis, 708 b). 
The definitive rectus internus of the Teleostei, in thus shift- 
ing its point of origin, passed dorsal to the efferent pseudobran- 
chial artery and dorsolateral to the internal carotid. The mem- 
branous tissues representing the presphenoid bolster were then 
pressed together in the median line by these muscles, and be- 
came the median vertical myodomic membrane, the internal 
carotid arteries still being enclosed in it, in a membranous canal, 
the homologue of the cartilaginous canals of Amia fused to 
form a single canal. The floor of the myodome of Amia be- 
came the horizontal myodomic membrane, which becomes 
adherent to the ventral! surface of the membranous pituitary 
sac and seems to end there. It, however, certainly continued, 
primarily, beyond that point and was continuous with the ven- 
tral end of the interorbital septum. The efferent pseudobran- 
chial arteries were necessarily pressed ventrally by the recti in- 
terni, and, losing their connections with the internal carotids, 
