380 
EDWARD PHELPS ALLIS. 
Heptanohus, four muscle-sheets superimposed one above the 
other, the two internal muscles being wholly independent 
of each other and of the external ones, because of the inter- 
vening branchial pouches, but the two external muscles being 
fused to a greater or less extent with each other in the mid- 
ventral line. It might then be assumed that these two 
external muscles belonged the one to the hyal and the other 
to the mandibular arch, and that they had, because of the 
abortion of the intervening branchial cleft, partially fused 
with each other, as the overlapping constrictores of the liyal 
and branchial arches of Mustelus and certain other Selachii 
have, and as has already been described. The conditions in 
Chlamydoselachus and Heptanclius can, however, equally well 
represent two different stages in the change of insertion of a 
hyal muscle from the branchial bar (ceratohyal) of its own 
arch to that (mandibula) of the mandibular arch, similar to 
the change of insertion that takes place in the dorsal portion 
of the muscle and has just been described. The first assump- 
tion requires the further assumption that the overlapping 
muscle of mandibular origin has wholly, or in large part, 
lost its primary innervation by the nervus trigeminus and 
secondarily acquired innervation by the nervus facialis ; and 
Chlamydoselachus, generally considered to be the most primi- 
tive of living Selachii, would present a more advanced stage, 
not only in the fusion of these two muscles but also in the 
secondary change of innervation, than any other selachian 
that I know of. The second of the two assumptions entails 
no secondary assumptions for its justification, excepting the 
readily acceptable one that part of a hyal muscle has secon- 
darily acquired insertion on a mandibular cartilage, and 
Chlamydoselachus, if a primitive fish, would naturally show 
an early stage in the process. The innervation of the muscles 
in Chlamydoselachus favours the second assumption. 
In this fish, Chlamydoselachus, in each of five specimens 
that I have examined, the nervus hyoideus facialis at first 
runs forward along the external surface of the posterior 
portion of the interhyoideus, and there gives off two or more 
