the cerato-hyal are attached a series of seven strong curved cylindrical rods, the branchiostegal rays. The first of these is the smallest and they increase in size up to the last. The four dorsal ones are attached to the outer surface of the cerato-hyal, the three ventral ones to its inner surface. Interposed between the hypo-hyals of the two sides is an unpaired somewhat triangular plate, the uro-hyal or basi-branchiostegal (fig. 12, 15).
The branchial arches.
The branchial arches are five in number and consist of the following parts on each side. The dorsal end is formed of the supra-pharyngeal bone, a large irregular bone covered ventrally with teeth of a fair size, and representing the fused pharyngo-branchials of the four anterior arches. Its external surface is continuous with four small epi-branchials which pass horizontally backwards and outwards. Their distal ends meet four long cerato-branchials which are directed forwards and inwards and form the principal part of the arches.
Each of the first three cerato-branchials articulates ventrally with a hypo-branchial, and the hypo-branchials of the two sides are united in the middle line by an unpaired basibranchial. The third hypo-branchial is much flattened. The fourth cerato-branchial is united by cartilage with the posterior surface of the third hypo-branchial, which it meets near the middle line.
The fifth arch consists only of the cerato-branchial, a wide structure covered with teeth and generally called the inferior pharyngeal bone.
The skeleton of the operculum consists of the same four bones as in the Salmon, namely the opercular, the infra-opercular, the pre-opercular and the sub-opercular. Of these the anterior bone, the pre-opercular, is the largest, while the infra-opercular is the smallest. The opercular has a facet for articulation with the hyomandibular.
2. The Appendicular Skeleton.
The Pectoral girdle.
This is of a highly specialised type. Membrane bones are greatly developed, and the cartilage bones, the scapula and coracoid, are much reduced in size and importance.
| 1. post-temporal. | 5. scapula. |
| 2. supra-clavicle. | 6. post-clavicle. |
| 3. clavicle. | 7. brachial ossicles. |
| 4. coracoid. | 8. dermal fin-rays. |
The largest bone in the shoulder girdle is the clavicle (fig. 13, 3), which is irregularly crescent shaped, thick in front and tapering off behind. To the outer side of its upper part is attached a thick cylindrical bone, the supra-clavicle, which passes upwards and is connected with a strong V shaped bone, the post-temporal. The apex of the V meets the supra-clavicle, the inner limb articulates with the epi-otic process, the outer with the parotic process. Projecting downwards from the upper part of the clavicle is a long bony rod, flattened proximally and cylindrical and pointed distally, this is the post-clavicle (fig. 13, 6).
The scapula (fig. 13, 5) is a small irregular plate of bone attached to the inner side of the middle of the clavicle. The coracoid[38] is a larger plate of similar character, irregularly triangular in shape, attached to the inner side of the clavicle immediately below the scapula. The scapula and coracoid bear the pectoral fin.
The Pectoral fins.
Each of these consists of four small irregular bones, the brachial ossicles (fig. 13, 7), bearing a series of about nineteen dermal fin-rays. The brachial ossicles represent the reduced and modified radiale and basalia of cartilaginous fish such as the dogfish. The fin-rays (fig. 13, 8) which form the whole external portion of the fin are long slender rods having essentially the same character as those of the unpaired fins.
The Pelvic girdle.
The pelvic girdle in the Cod as in other Teleosteans is entirely absent, its place being taken by the enlarged basi-pterygia of the fins.
The Pelvic fins.
These have a very anomalous position in the Cod, being attached to the throat in front of the pectoral girdle. Each consists of a basal portion, the basi-pterygium, and of a number of dermal rays. The basi-pterygium consists of an expanded ventral portion which meets its fellow below in the middle line, and to which the rays are attached, and of an inwardly-directed dorsal portion which also meets its fellow and is imbedded in the flesh. The rays are six in number and are long slender structures similar to those of the other fins.
CHAPTER VIII.
GENERAL ACCOUNT OF THE SKELETON IN FISHES[39].
EXOSKELETON.
The most primitive type of exoskeleton is that found in Elasmobranchs and formed of placoid scales; these are tooth-like structures consisting of dentine and bone capped with enamel, and have been already described (p. 4). In most Elasmobranchs they are small and their distribution is fairly uniform, but in the Thornback skate, Raia clavata, they have the form of larger, more scattered spines. In adult Holocephali and in Polyodon and Torpedo there is no exoskeleton, in young Holocephali, however, there are a few small dorsal ossifications.
The plates or scales of many Ganoids may have been formed by the gradual fusion of elements similar to these placoid scales, and often bear a number of little tooth-like processes. In Lepidosteus, Polypterus, and many extinct species, these ganoid scales, which are rhomboidal in form and united to one another by a peg and socket articulation, enclose the body in a complete armour. In Trissolepis part of the tail is covered by rhomboidal scales, while rounded scales cover the trunk and remainder of the tail. Acipenser and Scaphirhynchus have large dermal bony plates which are not rhomboidal in shape and do not cover the whole body. In Acipenser a single row extends along the middle of the back and two along each side.
The majority of Teleosteans have thin flattened scales which differ from those of Ganoids in being entirely mesodermal in origin, containing no enamel. There are two principal types of Teleostean scales, the cycloid and ctenoid. A cycloid scale is a flat thin scale with concentric markings and an entire posterior margin. A ctenoid scale differs in having its posterior margin pectinate. The Dipnoi have overlapping cycloid scales. The rounded scales of Amia and of many fossil ganoids such as Holoptychius are shaped like cycloid scales, but differ from them in being more or less coated with enamel. In Eels and some other Teleosteans the scales are completely degenerate and have almost disappeared. Some Teleosteans, like Diodon hystrix, have scales with triradiate roots from which arise long sharp spines directed backwards. These scales, which resemble teeth, contain no enamel; they become erect when the fish inflates its body into a globular form. Many Siluroids have dermal armour in the form of large bony plates which are confined to the anterior part of the body. In Ostracion the whole body is covered by hexagonal plates, closely united together.
The fin-rays are structures of dermal origin which entirely or partially support the unpaired fins, and assist the bony or cartilaginous endoskeleton in the support of the paired fins.
In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of the fins are, as a rule, about half