26. epi-hyal.
In a ventral view the cranium is seen to be chiefly covered by two large membrane bones, the parasphenoid (fig. 9, B, 9) behind, the vomer in front. A view of the posterior end differs from that of the cartilaginous cranium only in the fact that the end of the parasphenoid appears lying ventral to the basi-occipital.
The lateral view differs very markedly from that of the cartilaginous cranium, there being a great development of membrane bone in connection with the jaws and branchial apparatus. Lying dorsally are seen the median ethmoid, frontal, parietal, and supra-occipital as before. Lying external to the middle of the median ethmoid is seen the small nasal (fig. 11, 8), and below the hinder part is the lachrymal. The lachrymal (fig. 11, 9) forms the first of a series of seven small bones which surround the orbit forming the orbital ring. Of these the one lying immediately in the mid-ventral line of the orbit is the suborbital, while the one lying in the mid-dorsal line and attached to the frontal is the supra-orbital (fig. 11, 11). The orbit has a cartilaginous sclerotic in which are two small ossifications (fig. 11, 13) laterally placed.
Bones of the upper jaw.
The palato-pterygo-quadrate bar is in a very different condition from that of the dogfish, it is partially cartilaginous, partially converted into cartilage bone, partially overlapped by membrane bone. It is narrow in front but becomes much broader and deeper when followed back. Its anterior end forms the palatine which bears teeth, and in front is completely ossified, while behind the cartilage is only sheathed by bone.
Just behind the palatine the outer part of the cartilage is ossified, forming two small bones, the pterygoid and meso-pterygoid, while behind them is a larger, somewhat square bone, the meta-pterygoid (fig. 11, 15).
Below the meta-pterygoid is a tract of unossified cartilage, and then comes the quadrate (fig. 11, 18).
The lower angle of the quadrate bears a cartilaginous condyle with which the mandible articulates. In front of the palatine the cartilaginous snout is overlapped by three membrane bones, the jugal, maxilla and premaxillae.
The premaxillae (fig. 11, 20), the largest of these, overlaps the maxilla behind; both bones bear teeth. The jugal (fig. 11, 17) lies above the maxilla and overlaps it in front.
The lower jaw.
The lower jaw is a strong bar and is like the upper jaw, partly cartilaginous, forming Meckel's cartilage, partly ossified, and sheathed to a considerable extent in membrane bone.
The outer side and posterior end is ossified, forming the large articular (fig. 11, 21), but the condyle is cartilaginous and the anterior part of the articular forms merely a splint on the outer side of Meckel's cartilage, which extends beyond it for a considerable distance. The angle of the jaw just below the condyle is formed by a small angular (fig. 11, 22), and the anterior two-thirds of the jaw is sheathed in the large tooth-bearing dentary (fig. 11, 23).
The Hyoid arch.
The hyoid arch has a number of ossifications in it and is closely connected with the mandibular arch.
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.