316 
EDWARD PHELPS ALLIS. 
are said by Vetter to arise from the inner (proximal) end of 
the dorsal extrabranchial, but it is here said that it is the 
extrabranchial of the arch to which the constrictor belongs. 
The next distal fibres are said to arise from a small and thin 
tendon which perforates the musculus trapezius and the 
dorsal trunk muscles to have its insertion on the vertebral 
column, and the distal and larger part of the fibres to arise 
from a narrow tendinous aponeurosis which extends dorso- 
anteriorly from the top of the gill opening next posterior to the 
muscle. Such a linear aponeurosis is found related to both the 
dorsal and ventral ends of each of the first four gill openings, 
and each pair of aponeuroses is said to unquestionably repre- 
sent the lines where the dorsal and ventral portions of the 
overlapping outer edge of a long and tall branchial dia- 
phragm, such as is found in Heptanchus, has fused with the 
anterior (external) surface of the next posterior branchial 
diaphragm in order to form the small gill openings of 
Acanthias. It is said that, as a natural consequence of this 
method of formation of these aponeuroses, the extrabranchials 
(ausseren Kiemenbogen) lie immediately beneath them, firmly 
adherent to them. It is not definitely said to which arch the 
extrabranchial related to a given aponeurosis belongs, but 
it is said (loc. cit., p. 430) that each aponeurosis marks 
the limit between the outer, visible, posterior portion of each 
constrictor (Constr. super, s. str.) and the musculus inter- 
brancliialis of the same arch, this latter muscle being covered 
by the next anterior branchial diaphragm. It is accordingly 
evident that Vetter considered the extrabranchial that under- 
lies a given aponeurosis to belong to the posterior one of the 
two branchial diaphragms that have fused to form the 
aponeurosis. 
Each of the linear aponeuroses of Acanthias is thus said 
to be a persisting cicatrice formed along the line where two 
adjoining branchial diaphragms have fused with each other, 
and, that being the case, the cicatrice in each individual fish 
must evidently have been formed during the life of that 
particular fish, for that a so-formed cicatrice could have been 
