VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 339 
several figures, and not to the first arch, as shown in Gegen- 
baur’s (1872) figure of Galeus. The coracobranchiales I and II 
arise from the dorsal surface of the hypobranchial muscles, 
but the coracobranchiales III, IV, and V from the lateral 
surface of the pericardial chamber. 
An angular piece has been cut out of the proximal edge of 
each constrictor, as in Scyllium, to form the musculus 
adductor of the arch, and the cut ends of the fibres are 
nserted on the related epibranchial and ceratobranchial. 
In the large head of Triakis, the muscles were only super- 
ficially examined. 'The constrictores superficiales here, as in 
Mustelus, form a continuous muscle -sheet perforated by the 
first four gill-openings, and there are dorsal aponeuroses, 
but no ventral ones. The dorsal aponeuroses are not so 
well developed as in my specimen of Mustelus, certain of 
the superficial muscle strands of the constrictores crossing 
the aponeuroses, and others there being simply pinched, 
without being completely interrupted. The aponeuroses 
here seem to have been in part formed by the invasion of 
subdermal connective tissues, rather than by the interruption 
of the muscle substance of the fibres. Where the fibres of 
the hyal constrictor cross the extrabranchial of that arch 
the fibres are also partially interrupted, and the beginning 
of the formation of a linear aponeurosis is plainly evident ; 
and here there is no invasion of connective tissues. That 
part of each constrictor that lies dorsal to the dorsal extra- 
branchial of its arch, excluding the first branchial arch, 
joins, as in Mustelus, and even more markedly so, the mus- 
culus trapezius. 
Comparing the conditions in Scyllium, Mustelus, and 
Triakis, as above described, with those described by Vetter 
in Heptanchus, it is certain that the continuous muscle- 
sheet formed by the constrictores superficiales in Mustelus 
and Triakis has arisen by the fusion of the separate but 
overlapping constrictores of Scyllium and Heptanchus. To 
