VISCERAL ARCHES OF THE GNATHOSTOME FISH US. 371 
this muscle is derived from trunk myotomes and not from 
branchial ones. 
In Heptanchus (Vetter), Hexanchus (Huge), and Chlamy- 
doselachus the fibres of the constrictor superficial^ of the 
hyal arch have a nearly dorso-ventral course, but in most 
other Selachii that have been described the dorsal and ventral 
ends of this constrictor are directed more or less posteriorly, 
and it is probable that in all these latter fishes, as is certainly 
the case in my specimen of Mustelus, the distal (posterior) 
fibres cross, in their course, the extrabranchials of one or 
more of the branchial arches. In certain of these fishes the 
ventral fibres extend posteriorly nearly or quite to the ventral 
end of the shoulder-girdle. The fibres of the muscle may 
become tendinous where they cross the extrabranchials of 
the branchial arches, particularly the dorsal extrabranchials. 
They do not usually become tendinous where they cross the 
extrabranchials of their own arch, nor are they inserted on 
those extrabranchials, this doubtless being due to the absence 
of an overlapping branchial diaphragm and gill-pouch, and 
accounting for the absence of a musculus interbranchialis in 
this arch. In the middle of the length of the constrictor, 
opposite the hyomandibulo-ceratohyal articulation, the muscle 
fibres are, probably in all Selachii, interrupted by a more or 
less extensive aponeurosis. Dohrn describes this aponeurosis 
even in embryos, but it is evident that the fibres must here 
have been primarily continuous, and Ruge (1897, p. 224) 
says that the conditions in the adult Hexanchus warrant this 
conclusion. 
In the proximal edge of the constrictor there is, as in the 
branchial arches, a large angular incisure, and this incisure 
is filled by the articulating ends of the epihyal and ceratohyal, 
the cut ends of the fibres being inserted on those cartilages. 
Comparison with the conditions in the branchial arches would 
then seem to make it practically certain that a piece has here 
been cut out of this hyal muscle, as it has been cut out of the 
branchial muscles, and that the pieces so cut out of these 
several muscles were all serial homologues. If this be so. 
