EDWARD PHELPS ALLIS. 
3?4 
each constrictor. From this dorsal edge of the sheet delicate 
tendinous lines cross the external surface of the trapezius 
and extend to the dorsal edge of that .muscle, apparently 
representing, as already stated, the tendinous hands there 
described by Vetter in Heptanchus. 
There are no linear aponeuroses, either dorsal or ventral, 
related to the gill-openings, but the fibres of the proximal 
(anterior) half of the hyal constrictor are interrupted, as in 
many other Selachii, by an aponeurosis that lies approxi- 
mately in the line of the middle line of the gill-openings. 
This aponeurosis is attached anteriorly to the mandibular 
cartilages, and covers the articulating ends of the hyal 
cartilages. 
The muscle strands in the proximal (anterior) portion of 
the dorsal half of the hyal constrictor run antero-ventrally 
and are inserted on the ventral half or two-thirds of the 
hyomaudibula, the deeper fibres being shorter than the 
superficial ones and having their origins on the dorso- lateral 
edge of the chondrocranium. 'The proximal fibres of this 
constrictor thus form a musculus levator hyomandibularis 
with two heads of origin. The next distal (posterior) strands 
of the constrictor are inserted in the aponeurosis, just above 
described, that extends posteriorly from the articulating ends 
of the mandibular cartilages. The distal (posterior) strands 
traverse the branchial diaphragm of their arch and are 
continuous from the dorsal to the ventral end of the muscle. 
In the dorsal and middle parts of their lengths these distal 
strands have a nearly dorso- ventral course. Ventrally they 
spread posteriorly and extend nearly to the ventral end of 
the shoulder-girdle, there lying external (ventral) to the 
ventral end of the constrictor of the first branchial arch and 
external also to the liypobranchial muscles. The strands, 
excepting a few distal (posterior) ones, all reach the mid- 
ventral line, and form, with the musculus intermandibularis, 
a continuous superficial muscle-sheet extending to the sym- 
physis of the mandibulse. In the posterior three-fifths of this 
continuous sheet the strands of opposite sides are separated 
