VISCERAL ARCHES OE THE GNATHOSTOME FISHES. 825 
by a median aponeurotic line. In the anterior two-fifths of 
the sheet the strands of opposite sides are directly continuous 
with each other. 
A large bundle of the superficial fibres of the ventral 
portion of the hyal constrictor, composed of a number of 
muscle strands, separates from the deeper fibres and has its 
origin on the articular end of the mandibula. Beneath this 
bundle, and also for a certain distance anterior to it, the 
deeper fibres of the constrictor have their origins on the 
ceratohyal, and the so-formed musculus interhyoideus is 
connected with the adductor mandibulseby a tendinous fascia 
which passes internal to the large bundle of superficial fibres 
and is apparently the homologue of the tendinous fascia 
described by Vetter, in a similar but wholly superficial 
position, in Acanthias. Internal to the dorsal edge of this 
fascia the ligamentum mandibulo-hyoideum has its insertion 
near the dorsal end of the ceratohyal. Anterior to the large 
bundle of superficial fibres, the superficial fibres of the 
continuous muscle-sheet all have their origins on the ventral 
(morphologically posterior) edge of the mandibula, the line 
of attachment of the muscle beginning immediately anterior 
to the tendinous fascia just above described. The anterior 
portion of this musculus intermandibularis is innervated by a 
branch of the nervus mandibularis trigemini, and is hence of 
mandibular origin. The posterior portion is innervated by 
the nervus hyoideus facialis. The musculus interhyoideus 
extends to the mid-ventral line and is wholly separate from 
and independent of the superficial, intermandibularis layer of 
the sheet. It extends anteriorly slightly beyond the anterior 
end of the mid-ventral aponeurotic line that separates the 
constrictor fibres of opposite sides from each other, and is 
apparently not inserted in that aponeurosis. 
The muscle strands of the constrictor of each of the 
branchial arches all have an approximately dorso-ventral 
course, and they lie, throughout much the larger part of 
their length, in the related branchial diaphragm and upon 
the anterior (external) surface of the next posterior gill- 
