336 
EDWARD PHELPS ALLIS. 
lines prolonged of the fibres of the related portion of the 
musculus interbranchialis of the arch to which the extra- 
branchial belongs, the latter fibres being inserted on the 
opposite, ventral, edge of the extrabranchial. In the fourth 
branchial arch a considerable number of - the most proximal 
fibres of the interbranchialis cross the anterior (external) 
surface of the extrabranchial of their arch and join the fibres, 
above described, that appear to form part of the musculus 
trapezius, lying contiguous with them, along their anterior 
edge, on the lateral surface of the trapezius. This definitely 
shows that the fibres that are inserted on the dorsal edge of 
the extrabranchial are not parts of the trapezius, as they 
appear to be, but are the dorsal portions of certain muscle 
strands of the constrictor that have been cut in two by 
insertion on the extrabranchial, the part ventral to the extra- 
branchial forming the musculus interbranchialis and the part 
dorsal to the extrabranchial secondarily fusing with the 
trapezius. 
The musculus interbranchialis of each branchial arch has 
its origin from that portion of the dorsal extrabranchial of 
its arch that lies proximal to tbe point where it begins to 
curve antero-ventrally along the dorsal edge of the next 
posterior gill-pouch, and also from somewhat more than half 
of that part of the same extrabranchial that lies postero- 
ventral to the curve, and its dorsal edge is seen in fig. 9 
lying between the extrabranchial and the dorsal edge of 
the next anterior gill-pouch. The fibres distal to those 
thus inserted on the extrabranchial of the arch form the 
thin sheet of muscle fibres that had to be cut in order to 
turn the large muscle-sheet of the constrictores superficiales 
downward as shown in the figure. These distal fibres of 
each interbranchialis, thus here apparently inserted on the 
internal surface of the large muscle-sheet, must, in younger 
specimens, have formed the interbranchial portion of long 
and continuous muscle strands that passed over the anterior 
(external) surface of the extrabranchial of their arch and 
were continued, beyond that extrabranchial, to the dorsal 
