[Illustration: FIG. 121.—Side view of the cranium of a Salmon (_Salmo salar_). Most of the membrane bones and the jaws have been removed. The cartilage is dotted. _al.s_, Alisphenoid; _bo_, basioccipital; _bs_, basisphenoid; _eo_, exoccipital; _ep_, epiotic; _l.eth_, lateral ethmoid; _ol_, olfactory capsula; _op_, opisthotic; _o.s_, orbito-sphenoid; _pr.o_, prootic; _ps_, parasphenoid; _pt.o_, pterotic; _so_, supraoccipital; _sp.o_, sphenotic; _t.c_, trabecular cornu; _u.l.c_, _u.l.c_^2, first and second upper labial cartilages; _v_, vomer; II, foramen for the optic nerve. (From W. K. Parker.)] The roof and floor of the primary cranium is completed by {213}certain investing dermal bones (Fig. 123, A). A pair of large _frontal_ bones form the cranial roof, and also help to roof in the orbital cavities. Behind the frontals, and separated from each other by the supraoccipital, there is a pair of small _parietals_, and anterior to the frontals a median _dermal mesethmoid_. A small _nasal_ bone overlies each olfactory recess. Ventrally, the base of the cranium, from the basi-occipital to the prenasal region, is strengthened by a large _parasphenoid_ behind, and a much smaller _vomer_ in front, both of which lie in the roof of the mouth. The vomer is a tooth-bone, and probably the parasphenoid also. [Illustration: FIG. 122.—Vertical and longitudinal section of the cranium of _Salmo salar_, showing the right half of the cranial cavity. Cartilage is dotted. _f_, Frontal; _v′_, fat-containing cavity in the mesethmoid cartilage; V, VII, IX, X, foramina for the fifth, seventh, ninth, and tenth cranial nerves. Remaining reference-letters as in Fig. 121. (From W. K. Parker.)] The mandibular arch (Fig. 123, B) is more modified than that of the Dog-Fish. The palato-pterygo-quadrate bars, or primitive upper jaw, no longer meet in front beneath the cranial floor, but each separately articulates in front with the lateral ethmoid of its side. Although still partly cartilaginous each bar is largely replaced either by cartilage bones, or by bones which begin as membrane bones or as tooth-bones and complete their growth by invading the cartilage and becoming in part cartilage bones. Its anterior portion is formed by a _palatine_ bone which articulates with the lateral ethmoid, and the middle portion by a _pterygoid_ and a _mesopterygoid_ bone, while the hinder part is ossified above as a _metapterygoid_ and below as a _quadrate_. The latter articulates with the lower jaw. Functionally, however, the primitive upper jaw is now replaced by a secondary upper jaw, formed on each side by a series of tooth-bones, situated external to the former, and meeting in front of the prenasal region of the primary cranium (Fig. 123, A). The series includes a dentigerous _premaxilla_ and _maxilla_, and a small toothless, scale-like _jugal_ bone. Each half of the lower jaw (Fig. 123, A, B) consists of a rod-like Meckel's cartilage or primary lower jaw.