The hyomandibular (fig. 11, 24) is a large bone which articulates with a shallow groove lined by cartilage and formed partly in the pterotic, partly in front of it. The hyomandibular is overlapped in front by the meta-pterygoid, while below it tapers and is succeeded by a small area of unossified cartilage followed by the forwardly-directed symplectic which fits into a groove in the quadrate.
The unossified tract between the hyomandibular and symplectic is continuous in front with a strong bar, which remains partly cartilaginous and is partly converted into cartilage bone. The proximal part is ossified, forming the epi-hyal, the middle part forms the cerato-hyal (fig. 11, 27), in front of which is the small hypo-hyal. The hyoid arches of the two sides are united by the large tooth-bearing glosso-hyal (fig. 11, 29). Attached to the lower surface of the hyoid arch are a series of twelve flat branchiostegal rays (fig. 11, 35). Each overlaps the one in front of it, the posterior one being the largest. The branchiostegal rays of the two sides are united in front by an unpaired membrane bone, the basi-branchiostegal (fig. 11, 36).
Opercular bones. Behind the hyomandibular there is a large bony plate, the operculum, formed of four large membrane bones. The anterior of these, the pre-opercular (fig. 11, 33), is crescentic in shape, and with its upper end a small supratemporal (fig. 11, 34) is connected.
Behind the upper part of the pre-opercular is the largest of the opercular bones, the opercular proper. Its lower edge overlaps the sub-opercular, and both opercular and sub-opercular are overlapped by the infra-opercular (fig. 11, 32) in front. The infra-opercular is in its turn overlapped by the pre-opercular.
Branchial arches.
There are five branchial arches, the first four of which bear gill rays. Each of the first three consists of a shorter upper portion directed obliquely backwards and outwards, and a longer lower portion forming a right angle with the upper and directed obliquely forwards and inwards. The greater part of each arch is ossified.
The upper part of either of the first two consists of a short tapering pharyngo-branchial directed inwards, and of a long epi-branchial tipped with cartilage at both ends. The junction of the upper and lower parts is formed by a cartilaginous hinge-joint between the epi-branchial and cerato-branchial. The cerato-branchial is a long bony rod separated by a short area of cartilage from the hypo-branchial, which is succeeded by the basibranchial meeting its fellow in the middle line. The fourth arch has a short epi-branchial and no ossified pharyngo-branchial, while the fifth is reduced to little more than the cerato-branchial, which bears a few teeth on its inner edge. All the branchial arches have projecting from their surfaces a number of little processes which act as strainers. The first and fourth arches have one series of these, the second and third have two.
THE SKULL OF THE CODFISH[37].
A full description having been already given of the Salmon's skull, that of the Codfish will be described in a briefer manner. The skull is very fully ossified, and the great number of plate-like bones render it a very complicated structure.
The Cranium.