The Lower Jaw.—The lower jaw or mandible is also complex, consisting of two divisions or rami, right and left, joined in front by a suture. The anterior part of each ramus is formed by the dentary bone (30), which carries the teeth. Behind this is the articular bone (28), which is connected by a joint to the quadrate bone (19). At the lower angle of the articular bone is the small angular bone (29). In many cases another small bone, which is called splenial, may be found attached to the inner surface of the articular bone. This little bone has been called coronoid, but it is doubtless not homologous with the coronoid bone of reptiles. In a few fishes, Amia, Elopidæ, and certain fossil dipnoans, there is a bony gular plate, a membrane bone across the throat behind the chin on the lower jaw.
The Suspensorium of the Mandible.—The lower jaw is attached to the cranium by a chain of suspensory bones, which vary a good deal with different groups of fishes. The articular is jointed with the flat quadrate bone (19), which lies behind the pterygoid. A slender bone passes upward (18) under the preopercle and the metapterygoid, forming a connection above with a large flattish bone, the hyomandibular (17), which in turn joins the cranium. The slender bone which thus keys together the upper and lower elements, hyomandibular and quadrate, forming the suspensorium of the lower jaw, is known as symplectic (18). The hyomandibular is thought to be homologous with the stapes, or stirrup-bone, of the ear in higher animals. In this case the symplectic may be homologous with its small orbicular bone, and the malleus is a transformation of the articular. The incus, or anvil-bone, may be formed from part of Meckel's cartilage. All these homologies are however extremely hypothetical. The core of the lower jaw is formed of a cartilage called Meckel's cartilage, outside which the membrane bones, dentary, etc., are developed. This cartilage forms the lower jaw in sharks, true jaw-bones not being developed in these fishes. In lampreys and lancelets there is no lower jaw.
Membrane Bones of Face.—The membrane bones lie on the surface of the head, when they are usually covered by thin skin and have only a superficial connection with the cranium. Such bones, formed of ossified membrane, are not found in the earlier or less specialized fishes, the lancelets and lampreys, nor in the sharks, rays, and chimæras. They are chiefly characteristic of the bony fishes, although in some of these they have undergone degradation.
The preorbital (49) lies before and below the eye, its edge more or less parallel with that of the maxillary. It may be broad or narrow. When broad it usually forms a sheath into which the maxillary slips. The nasal (51) lies before the preorbital, a small bone usually lying along the spine of the premaxillary. Behind and below the eye is a series of about three flat bones, the suborbitals (50), small in the striped bass, but sometimes considerably modified. In the great group of loricate fishes (sculpins, etc.), the third suborbital sends a bony process called the suborbital stay backward across the cheek toward the preopercle. The suborbital stay is present in the rosefish. In some cases, as in the gurnard, this stay covers the whole cheek with a bony coat of mail. In some fishes, but not in the striped bass, a small supraorbital bone exists over the eye, forming a sort of cap on an angle of the frontal bone.
The largest uppermost flat bone of the gill-covers is known as the opercle (25). Below it, joined by a suture, is the subopercle (26). Before it is the prominent ridge of the preopercle (24), which curves forward below and forms a more or less distinct angle, often armed with serrations or spines. In some cases this armature is very highly developed. The interopercle (27) lies below the preopercle and parallel with the lower limb.
Branchial Bones.—The bones of the branchial apparatus or gills are very numerous and complex, as well as subject to important variations. In many fishes some of these bones are coossified, and in other cases some are wanting. The tongue may be considered as belonging to this series, as the bones of the gills are attached to its axis below.
In the striped bass, as in most fishes, the tongue, gristly and immovable, is formed anteriorly by a bone called the glossohyal (37). Behind this are the basihyals (36), and still farther back, on the side, is the ceratohyal (35). To the basihyals is attached a bone extending downward and free behind the urohyal (38). Behind the ceratohyal and continuous with it is the epihyal (34), to which behind is attached the narrow interhyal (33). On the under surface of the ceratohyal and the epihyal are attached the branchiostegals (39). These are slender rays supporting a membrane beneath the gills, seven in number on each side in the striped bass, but much more numerous in some groups of fishes. The gill membranes connecting the branchiostegals are in the striped bass entirely separate from each other. In other fishes they may be broadly joined across the fleshy interspace between the gill-openings, known as the isthmus, or again they may be grown fast to the isthmus itself, so that the gill-openings of the two sides are widely separated.
The Gill-arches.—The gills are attached to four bony arches with a fifth of the same nature, but totally modified by the presence of teeth, and very rarely having on it any of the gill-fringes. The fifth arch thus modified to serve in mastication instead of respiration is known collectively as the lower pharyngeals (46). Opposite these are the upper pharyngeals (45).
The gill-arches are suspended to the cranium from above by the suspensory pharyngeal (44). Each arch contains three parts—the epibranchial (43), above, the ceratobranchial (42), forming the middle part, and the hypobranchial (41), the lower part articulating with the series of basibranchials (40) which lie behind the epihyal of the tongue. On the three bones forming the first gill-arch are attached numerous appendages called gill-rakers (47). These gill-rakers vary very greatly in number and form. In the striped bass they are few and spear-shaped. In the shad they are very many and almost as fine as hairs. In some fishes they form an effective strainer in separating the food, or perhaps in keeping extraneous matter from the gills. In some fishes they are short and lumpy, in others wanting altogether.
