332 
EDWARD PHELPS ALLIS. 
on the posterior surface of that pouch (Fig. 3). The action 
of these fibres, although feeble, must be to retract and 
constrict the related gill-opening. 
The dorsal and ventral edges of the gill-pouches are 
wholly free, excepting where they are each attached, as 
already described, to the dorsal extrabranchial of the next 
anterior arch and to the internal surface of the constrictor 
of that arch, and there are, as already stated, no linear 
aponeuroses in any of the constrictores. It is accordingly 
impossible that the dorsal edges of these pouches, in Scyllium, 
have been formed by the partial fusion of the edges of a 
taller gill-opening, as is maintained by Vetter, Tiesing, and 
Ruge for the Selachii examined by them. 
The coracomandibularis, coracohyoideus, and coraco- 
branchialis I all have their origins on the musculus coraco- 
arcualis communis, the other coracobranchiales having their 
origins on the ventral end of the shoulder-girdle. The 
coracomandibularis is inserted on the mandibula close to 
the symphysis, the coracohyoideus on the anterior edge of the 
ventral surface of the basihyal, the coracobranchialis I on the 
dorsal surface of the postero-lateral process of the basihyal, 
the coracobranchiales II-IV each on the hypobranchial of 
the related arch, and the coracobranchialis V on the cerato- 
brancliial of its arch. There are arcualis, but no interarcualis 
muscles in this fish, each arcualis being a stout muscle, 
evidently primarily continuous with the constrictor super- 
ficialis of its arch, but lying ventral to the vena jugularis 
instead of lateral to that vein. The primitive constrictor, in 
growing dorsally, seems to have been split into two parts 
when it encountered the vein, one passing ventral and the 
other dorsal to it. 
In the small specimen of Mustelus (PI. 22, figs. 8-10) the 
constrictores superficiales of the hyal and branchial arches 
together present, in lateral view, the appearance, ventral as 
well as dorsal to the gill-openings, of a single continuous 
muscle-sheet, the sheet being perforated by the first four gill- 
