VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 337 
edge of the muscle. They formed the middle portion, 
approximately, of each branchial constrictor, and as the 
constrictores of this fish overlapped each other to a con- 
siderable extent, they must have lain internal to certain 
strands of the next anterior constrictor, and they and the 
overlying fibres both became tendinous where they crossed 
the extrabranchial. If, then, those portions of these deeper 
fibres, or strands, that primarily lay dorsal to the extra- 
branchial and the related secondarily formed aponeurosis 
did not abort, they must still persist as a deeper component 
of the large muscle-sheet. 
Ventral to the gill-openings the conditions, aside from the 
absence of linear aponeuroses, differ in minor details only 
from those dorsal to the gill-openings. Here each musculus 
interbranchialis, excepting only a few proximal fibres, is 
inserted, its full length, on the ventral extrabranchial of 
its arch, and in each of the branchial arches of my preserved 
specimen there is a slight fold in the muscle just before it 
reaches the extrabranchial ; the muscle passing, from above, 
posterior (internal) to the extrabranchial, then turning 
dorsally upon itself, and then again ventrally to its insertion 
on the dorso-anterior edge of the extrabranchial (Fig. 10). 
From the opposite, postero-ventral edge of the extrabranchial 
a corresponding portion of the fibres of the constrictor of each 
arch have their origins, and running ventro-posteriorly immedi- 
ately join, on its internal surface, the continuous muscle-sheet 
formed by the constrictores superficiales of the several arches. 
These ventral fibres of the constrictor of each arch thus form 
a component part of the large muscle-sheet, and they here lie 
internal to certain fibres of the more anterior arches; but, after 
crossing the next posterior extrabranchial, they lie between 
those fibres and certain fibres of the next posterior arch, 
internal to the ones and external to the others. Here there 
can be no question as to the persistence of these ventral 
portions of the fibres of each arch, for they are not here 
interrupted by linear aponeuroses. 
A few of the most proximal strands of the interbranchialis 
