366 
EDWARD PHELPS ALLIS. 
branchial arches, in the hyal and mandibular arches, or fused 
with the neurocranium, not only in all living Teleostomi but 
also in most, if not all, higher vertebrates. If this con- 
clusion is correct, and if these extrabranchials are simply 
modified branchial rays, as is generally accepted, then the 
early ancestors of the Teleostomi must have possessed those 
rays. But I have, since the publication of the paper above 
referred to, found that Braus (1906) concludes, from con- 
ditions found in embryos of Heptanchus, that the extra- 
branchials belong to an independent category of skeletal 
pieces. If this be so, it then seems probable that the early 
ancestors of the Teleostomi possessed these particular carti- 
lages, but not the ordinary branchial rays with which they 
a, re usually associated. 
The branchial muscles of the Selachii seem to be more primi- 
tive than those of any other of the gnathostome fishes. When, 
in the ancestor of the Selachii, the branchial arches acquired 
positions oblique to the axis of the body, and later, or at the 
same time, acquired the sigma form, the proximal edge of the 
simple dorso-ventral Constrictor of each arch slipped, in the 
middle of its length, over the anterior (actually lateral) 
surface of the branchial bar of its arch, and the fibres of the 
muscle, where they crossed the branchial bar, there first 
became tendinous, by the interruption of their muscular 
substance, and were then later cut through by acquiring' 
insertion on the bar. A triangular piece was thus cut out of 
the proximal edge of the muscle, and became the adductor 
of the arch. The gill-pouch anterior to the arch, pressing 
against the anterior surface of that part of the constrictor 
that remained external to the branchial bar, first caused a 
simple thinning of the muscle. The dorsal and ventral rays 
of the branchial series were then modified, as extrabranchials, 
in supporting relations to this thinned part of the muscle, 
or these extrabranchials were otherwise and independently 
developed for the same purpose, and at certain places in the 
lines where the muscle passed over these cartilages, it again 
became tendinous, or acquired insertion on the cartilages. 
