390 
EDWARD PHELPS ALLIS. 
have been primarily, in the mandibular arch as in the hyal 
and branchial arches, a single continuous constrictor muscle 
that had a dorso-ventral extent equal to that of the con- 
strictor of the hyal arch. Branchial rays also probably 
primarily existed in this arch as in the more posterior arches, 
for remnants of them are said to be still found in certain 
recent fishes. Branchial lamellee were then quite probably 
also developed in this arch as in the more posterior ones, and 
were probably found on the anterior as well as the posterior 
surface of the arch. 
The conditions in this mandibular arch would then have 
been similar to those in the hyal and branchial arches, and, 
such being the case, there seems no good reason why, when 
the visceral arches all began to assume a position oblique to 
the axis of the body, a small adductor muscle should not have 
been cut out of the proximal edge of the constrictor of this 
arch, as it is said to have been cut out of the constrictores of 
the branchial arches, and as I assume that it was also cut out 
of the constrictor of the hyal arch. 
When the hyomandibular cleft later became reduced to the 
small existing spiracular canal, the mandibular branchial 
diaphragm, which must primarily have existed as in the hyal 
and branchial arches, would necessarily have gradually 
ceased to be formed excepting as it may still be represented 
in some part of the anterior wall of the spiracular canal. 
Because of this gradual reduction and final almost complete 
disappearance of the branchial diaphragm of this arch, the 
middle portion of the long constrictor superficialis of the 
arch was necessarily forced over onto the anterior (lateral) 
surface of the cartilaginous bar of the arch, and it carried 
with it the nerve of the arch, and probably also the 
anterior efferent artery of the arch (Allis, 1916), this nerve 
and artery primarily lying' anterior to the constrictor muscle, 
as they actually do in the branchial arches of living 
Selachii. The afferent mandibular artery and the branchial 
rays, both lying posterior to the constrictor muscle, were not 
so carried forward, and retained their primitive positions on 
