VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 321 
hyal arch of this fish is in part inserted on the extrabranchial 
of the first branchial arch, which leads one to suppose that 
that extrabranchial lies beneath the linear aponeurosis related 
to the first gill opening. 
The musculi interbranchiales of Mustelus are said by Tiesing 
to be practically as described by Vetter in Acanthias. 
These several descriptions of these visceral-arch muscles 
thus, as already stated, do not give a clear and concise idea 
of the conditions in these fishes, and I have accordingly, as 
already stated, had dissections made of such specimens of the 
Selachii as I have at my disposal, which specimens consist of 
a single already partly dissected head of Triakis, a 42 cm. 
specimen of Scyllium canicula, and a 43 cm. specimen of 
Mustelus (species unknown) . The accompanying figures show 
the muscles as found in the two last-named specimens. The 
musculi constrictores superficiales, interbranchiales, and 
coracobranchiales were alone particularly considered in the 
dissections, but other muscles are also shown in the figures. 
The constrictor superficialis of each arch will be considered 
as a single continuous muscle, instead of as two separate 
muscles, one dorsal and the other ventral. I retain the term 
constrictor superficialis, but, as there is no constrictor pro- 
fundus, it seems a needless distinction. 
In my specimen of Scyllium (figs. 1-7), those portions of 
the constrictores superficiales of the hyal and first four 
branchial arches that lie dorsal to the gill openings appear 
to form, in lateral view, a single continuous muscle-sheet. 
Immediately dorsal to the gill openings the lines of separation 
between adjoining constrictores are apparent, and, starting 
from there, each constrictor can be easily lifted off the next 
posterior one excepting at its dorso-posterior corner. At 
that corner each constrictor is inserted on the external 
surface of the next posterior one, but elsewhere its distal 
edge simply overlaps and is closely applied to that muscle. 
Ventral to the gill openings the constrictores are less closely 
applied to each other, the lines of separation between 
