VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 341 
noted that in my specimen of a small but adult Scyllium it 
would be impossible to, cut a section that could show the 
conditions given by Dohrn in embryos of that fish. The 
constrictores superficiales of tliis fish are so nearly dorso- 
ventral in position in their dorsal portions that no one of 
them overlaps more than the next posterior muscle, the gill- 
pouches elsewhere intervening and separating the muscles ; 
and in their ventral portions the constrictores do not at any 
point fuse with each other, as Dohrn definitely shows them 
in his sections. 
In the adult specimens of Acanthias and Scymnus that 
were examined by Vetter and Marion, conditions strictly 
similar to those above described in Mustelus and Triakis 
must certainly have existed, but they were misinterpreted 
by those authors. Minor differences, however, apparently 
exist, as in Acanthias, where it would seem, from Vetter's 
descriptions, as if the dorsal fibres of the constrictor of a 
given arch, had largely aborted after they had crossed the 
extrabranchial of the next posterior arch, had still more 
largely aborted after crossing the extrabranchial next pos- 
terior to that one, and had wholly aborted on reaching the 
third posterior extrabranchial. 
The constrictores superficiales of Mustelus, Triakis, Acan- 
thias, and Scymnus, and of all other Selachii where the 
conditions are similar, thus forming a continuous muscle- 
sheet which is subdivided, by the transverse aponeuroses, 
into what appear like several separate segments, the ques- 
tion of the innervation of these several segments becomes 
important. Vetter says that he could not satisfactorily 
determine the innervation of these muscles in the branchial 
arches either of Acanthias or Scymnus, but in the hyal arch 
of those fishes he limits the distribution of the branches of 
the nervus facialis strictly to the constrictor of that arch, as 
that constrictor is defined by him. Tiesing says that, in 
Mustelus, the branches of the nervi glossopharyngeus and 
vagus are distributed only to those segments of the dorsal 
portions of the constrictores of that fish that are included 
