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-quad rates 

 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. 



P 



b.br 



ihk.c 



FIG. 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 ; i.hy, inter-hyal ; i.op, inter-operculum ; lys, ligaments connecting 

 the palato-quadrate cartilage with the hyomandibular ; mk.c, Meckel's cartilage ; 

 mx, maxilla ; op, operculum ; pa, palatine ; pa.q, palato-quadrate ; ps.l, pre- 

 spiracular ligament ; q, 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 hyostylic. The hyoid 

 and branchial arches are only partially ossified. 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 



VOL. VII Q 



