344 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
The nature of the add. tentaculi has already been considered, it 
being merely a separation of the deeper fibres of the add. mand. 
The lev. arc. pal. is plainly derived from a constrictor, but its: 
function has been changed by the development of osseous structures, 
so that instead of assisting in the contraction of the pharyngeal 
cavity, it enlarges it by raising the hyomandibular apparatus, ete. 
The reason why a trigeminal muscle should act as the opponent of 
muscles supplied by the seventh nerve, is that the forward growth 
superficially of the hyoidean muscles was prevented by the presence 
in primitive forms of the spiracle. The dil. opere. is evidently a 
portion of the lev. arc. pal. adapted to the necessities of the opercular 
apparatus. The incongruity between its action and its innervation 
is even more apparent than in the lev. arc. pal., but is explicable in 
the same way as Vetter has pointed out. 
The intermandibularis is without doubt the representative of the 
most anterior ventral portions of the Selachian constrictor. It is 
supplied by both the fifth and the seventh nerve, and instead, there- 
fore, of being assigned to the group of muscles supplied by the fifth 
nerve, as Vetter has done, it must be considered as representing the 
ventral portion of a constrictor layer lying between the palatine and 
mandibular and the mandibular and hyoidean arches. The anterior 
moiety of such a layer would be supplied by the fifth, and the pos- 
terior by the seventh nerve. In the Teleosts this layer has con- 
tracted in breadth very much, until it forms merely a narrow band 
between the extremities of the mandibular arch, but, with the grad- 
ual narrowing, there has been, so to speak, a corresponding lengthen- 
ing out of the innervating branch from the facialis and a shortening 
of that from the trigeminus, so that even when limited to the mandi- 
bular arch it still possesses its hyoidean nerve. 
Just as all the muscles of the mandibular arch (2.e., those supplied 
by the fifth nerve), are derived from a constrictor, so are all those of 
the hyoid arch, (i.e., those supplied by the seventh nerve.) The add. 
arc. pal. has apparently an abnormal position, extending between the 
skull and the palatine, metapterygoid and hyomandibular, thus com- 
ing into relation not only with the arches to which it belongs but also 
with the arch in front of it. The only explanation to be given for 
this is that the muscle has extended its insertion forwards as neces- 
sity required it. In Amiurus, owing to the necessity for motion of 
the palatine for the purpose of erecting (abducting) the tentacle sup- 
