VIII SKULL 225 
dermal bones invest the dorsal surface of the chondrocranium, 
and only to a limited extent correspond with the less numerous 
membrane bones of the Salmon. ‘To these are added a series of 
circum-orbital bones and a large parasphenoid. Undoubtedly the 
most striking feature in these Fishes is the primitive character 
of the upper jaw. In Polyodon (Fig. 131) the palato-quadrates 
are wholly cartilaginous, and, as in the Dog-Fish, they meet 
in front beneath the basis cranii, where the two are connected 
by ligament. The secondary upper jaw is but feebly developed, 
and is represented on each side by a thin splint-like maxilla in 
relation with the outer surface of each palato-quadrate cartilage, 
Fie, 131.—Lateral view of the primary and secondary upper and lower jaws of Polyodon. 
b.br’, First basi-branchial ; ch, cerato-hyal; d, dentary ; hy.h, hypo-hyal ; hy.m, 
hyomandibular ; ¢.hy, inter-hyal ; z.op, inter-operculum ; /ys, ligaments connecting 
the palato-quadrate cartilage with the hyomandibular ; mk.c, Meckel’s cartilage ; 
mz, maxilla; op, operculum; pa, palatine; pa.g, palato-quadrate ; ps./, pre- 
spiracular ligament ; g, quadrate cartilage ; sym, symplectic. (From Bridge.) 
which meets its fellow in front. There are no premaxillae. The 
lower jaw is also very primitive. Meckel’s cartilages are per- 
sistent, and except for a mento-Meckelian bone on each side, they 
are unossified, although membrane bones representing dentary and 
splenial elements are present. The skull is hyostyhe. The hyoid 
and branchial arches are only partially ossitied. Each opercular 
fold is supported by an operculum and an interoperculum, and both 
of these retain somewhat the shape of the cartilaginous hyoidean 
rays which they have replaced. In the Sturgeon (Fig. 130) the 
upper jaw is greatly modified in relation with the singular mouth 
of this Fish. The palato-quadrate cartilages meet not only in 
front, but also along their dorsal margins, and, with the help of 
the similarly opposed and somewhat fragmentary metapterygoid 
NOS VLE Q 
