VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 333 
openings and bounded posteriorly by the fifth opening. Both 
dorsal and ventral to these gill-openings, the muscle strands 
of the continuous sheet incline posteriorly at a marked angle 
to the vertical line, this inclination being greater dorsal to 
the openings than ventral to them, and greater for the 
posterior strands of the sheet than for the anterior ones. A 
narrow aponeurotic line extends dorso anteriorly from the 
dorsal end of each of the four gill-openings that perforate 
the sheet, but there are no corresponding ventral aponeuroses. 
Most of the muscle strands on either side of the dorsal 
aponeurotic lines are juxtaposed, and faint tendinous lines 
cross the aponeuroses and connect them, this strongly sug- 
gesting that the muscle strands were here primarily con- 
tinuous, and that the aponeuroses are simply tendinous 
formations, of secondary origin, that interrupt them. The 
muscle-sheet, in situ, accordingly presents the appearance of 
being formed by a series of contiguous constrictores, one 
hyal and four branchial, fused by their adjoining edges both 
dorsal and ventral to the gill-openings but separated from 
each other as they pass between those openings. The sheet 
is, however, not so formed, as will be immediately shown, but 
this superficial appearance, as it applies to the ventral part 
of the sheet, is doubtless what led Tiesing to say that, in this 
fish, the entire ventral constrictor of each arch traverses the 
related branchial diaphragm and is continuous with the dorsal 
constrictor. 
The anterior edge of the dorsal portion of the sheet is 
considerably thickened, and this thickened portion is almost 
completely differentiated, as in Scyllium, as a levator hyo- 
mandibularis with two heads of origin, one arising along the 
line of the latero-sensory canal of the body, continuous with 
the fibres of the posterior portion of the sheet, and the other 
from the dorso-lateral edge of the chondrocranium, as Tiesing 
has already stated. Both portions are inserted, together, on 
the ventral end of the hyomandibula. 
If that part of the muscle-sheet that lies posterior to this 
levator hyomandibularis be cut along its line of dorsal attach- 
VOL. 63 , PART 3 . NEW SERI KS. 24 
