340 
EDWARD PHELPS ALLIS. 
what extent these overlapping and fused muscles have each 
been preserved, or have aborted, is problematical; but it is 
certain that the several sections of the continuous muscle- 
sheet that are included between each two of the series of 
dorsal aponeuroses each contains elements derived from at 
least two adjoining arches. In my specimen of Mustelus 
certain of the fibres of the constrictor of the hyal arch even 
cross, in their dorso-posterior course, all of the dorsal extra- 
branchials of the fish, including the somewhat rudimentary 
extrabranch ial of the hyal arch. Those portions of the fibres 
of the constrictores of Mustelus and Triakis that lie ventral 
to the ventral extrabranchials are not interrupted by linear 
aponeuroses, and there is hence no reason to suppose that 
they there aborted, or even became tendinous. They must 
simply have joined the overlying fibres of the continuous 
muscle-sheet and persisted as part of it. There is, however, 
no noticeable evidence of any thickening of the sheet at these 
places. The muscle-sheet is, on the contrary, much thicker 
in its anterior portion, where there is no overlapping of these 
muscles, than in its posterior portion, where this overlapping 
takes place. 
This interpretation of the constrictores in these several 
fishes, based wholly on anatomical investigation, finds un- 
expected confirmation in Dohrn’s figures of sections of 
embryos said to be of Scyllium canicula (1884, Figs. 1-4, 
PI. 7). In those figures Dohrn shows the constrictores super- 
ficiales overlapping each other to such an extent that three, 
or even four, of them may be superimposed the one above 
the others, and certain of them are even shown fused with 
each other to form a single muscle-sheet. How sections of 
a selachian, with the constrictor muscles arranged as described 
by Dohrn, Vetter, and others, could be sectioned so as to 
show these muscles in this relation to each other has heretofore 
been to me incomprehensible, but sections of a fish in which 
these muscles were as I have above described and interpreted 
them in Mustelus could easily be sectioned to show them as 
given in Dohrn’s figures. It is, however, to be particularly 
