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. [Illustration FIG. 13. THE RIGHT HALF OF THE PECTORAL GIRDLE AND RIGHT PECTORAL FIN OF A COD (_Gadus morrhua_) × 1/2 (Brit. Mus.). 1. post-temporal. 2. supra-clavicle. 3. clavicle. 4. coracoid. 5. scapula. 6. post-clavicle. 7. brachial ossicles. 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.