VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 829 
of this depression corresponds to the outer edge of the pouch, 
and not, as might have been expected, to the outer edges of 
the branchial lamellae; the lamellae lying within the pouch 
and not extending to its outer edges. The constrictor is 
thinner in the depression than it is immediately beyond it. 
This, and the insertion of certain of the fibres on the dorsal 
and ventral extrabranchials of the arch, are the only indica- 
tions of the differentiation of a musculus interbranchialis. 
Dorsal extrabranchials are found in the hyal and first four 
branchial arches, but ventral extrabranchials only in the first 
three branchial arches. The dorsal extrabran chial of the 
hyal arch is a small rod of cartilage lying near the outer, 
distal ends of the branchial rays, and was not found by 
either White (1896) or Fiirbringer (1903, p. 432) in the 
specimens examined by them. The dorsal and ventral extra- 
brachials of the branchial arches lie along the dorsal and 
ventral edges, respectively, of the depression, just above 
described, formed on the opposite side of the constrictor of 
their arch by the pressure of the next anterior gill-pouch. 
Each of the four dorsal extrabranchials, running dorso- 
anteriorly from its distal end, reaches the curved dorsal edge 
of the gill-pouch next posterior to it immediately proximal to the 
highest point of that edge, and there turns sharply v en trail v 
and but slightly anteriorly over that edge, and then expands 
into a short spatula-shaped end (Fig. 7) which lies against 
the posterior surface of the gill-pouch. The dorsal end of 
each of these dorsal extrabranchials is thus crook-shaped, 
the crook lying close to the dorsal edge of the constrictor of 
the arch and embracing the dorsal edge of the next posterior 
gill-pouch, that pouch there being firmly attached to the 
extrabranchial by connective tissues. The dorsal half of 
the flat spatula-shaped end of the first extrabranchial lies 
against the external surface of the musculus trapezius, closely 
attached to it by connective tissues, and its ventral half 
against the external wall of the vena jugularis. The corre- 
sponding ends of the other three dorsal extrabranchials lie 
upon and are strongly attached to the musculus trapezius 
