VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 335 
muscle substance of the fibres, because of pressure against 
the branchial bars, and the consequent formation of a ten- 
dinous membrane by the united sarcolemmae of the fibres so 
interrupted. This same formation of a tendinous interval, 
with later insertion, might evidently take place along the 
lines where the muscle fibres of the primitive constrictores 
crossed any other skeletal element, and in my specimen of 
Mustelus it has quite certainly taken place where the fibres 
of the continuous muscle-sheet crossed the dorsal extra- 
branchials, the process, with these particular fibres, not being 
carried to the point of section of the fibres with accompanying 
insertion on the extrabranchials, while with other fibres this 
section and insertion has taken place. 
The distal end of each extrabranchial lies, as in Scyllium, 
on the anterior (external) surface of the gill-pouch next 
posterior to it, and when the dorsal edge of that gill-pouch 
passes its highest point and turns antero-ventrally toward 
the pharyngeal opening of the pouch, the extrabranchial also 
curves antero-ventrally and lies along the edge of the pouch. 
In each of the branchial ai’ches the proximal end of the extra- 
branchial then there expands into a flat and thin plate which 
projects ventrally along the posterior surface of the gill- 
pouch, there lying either between the pouch and the vena 
jugularis or between the pouch and the musculus trapezius, the 
exact relation of each extrabranchial to the vena jugularis and 
musculus trapezius not being traced. In the hyal arch there is 
no plate-like expansion of the proximal end of the extrabran- 
chial, this extrabriinchial being, as in Scyllium, simply a slender 
rod of cartilage. In the second, third, and fourth branchial 
arches each extrabranchial, at the point where it crosses the 
dorsal edge of the related gill-pouch and curves antero-ven- 
tfc-ally along that edge, is somewhat embedded in the musculus 
trapezius, and it there gives insertion to what appear to be 
superficial fibres of the trapezius, the number of these fibres 
increasing progressively from the second to the fourth arches. 
The fibres so inserted on each extrabranchial lie not only 
parallel to the fibres of the musculus trapezius, but also in the 
