VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 361 
between the epibranchial and ceratobranchial in each of the 
first three branchial arches. These fibres being found only in 
a particularly large specimen of this fish, does not favour the 
view that they are persisting fibres of a muscle that is in 
process of reduction ; for one would naturally expect to find 
such a muscle relatively the more developed the younger the 
fish. In Ameiurus, Wright (1885) did not find any of these 
muscles, and they are said by Pollard (1892) not to be found 
in Polypterus. In the Dipneusti, also, they are apparently 
not found, for Fiirbringer (1904) makes no mention of them 
in his descriptions of those fishes. 
In Amia, I described (Allis, 1897) two adductores arcuum 
branchialium, one related to the fourth aud the other to the 
fifth branchial arch. The fourth adductor arises from the 
internal surface of a posteriorly projecting process of the 
fourth epibranchial, and is inserted on a similar process of 
the fourth ceratobranchial, the muscle thus lying on the 
posterior surface of the branchial bar. The fifth adductor 
extends from the opposite side of the process of the fourth 
ceratobranchial just above mentioned to the fifth cerato- 
branchial, lies somewhat on the posterior surfaces of those 
two cartilages, and is in part continuous, ventrally, with the 
transversus ventralis posterior. The branch of the vagus 
that innervates these muscles passes, in each case, over the 
posterior edge of the related branchial bar. 
In Polyodon, Danforth (1913) finds an adductor arcus 
brauchialis in each of the first four branchial arches. Each 
muscle arises from the flat posterior surface of the related 
epibranchial, the surface of origin not approaching the margin 
of the cartilage at any point, and the muscle is covered by a 
tough aponeurotic sheet which binds it to the cartilage and 
also serves as a secondary basis of origin. Running ventro- 
laterally each muscle passes between the epibranchial and 
ceratobranchial of its arch and is inserted on the anterior 
surface of the latter cartilage. Branches of the nerrus vagus 
of the related arch are sent to the muscle, passing, in each 
case, over the posterior edge of the related epibranchial. 
