As to opercular bones, Acipenser has a large operculum: the Dipnoi have opercular and interopercular bones; the inter-operculum is absent in Polypterus and the prae-opercular fused with the squamosal: the prae-operculum is very small in Lepidosteus, the interoperculum of great size. Amia resembles a Teleostean. Amia has a single, Polypterus a double jugal bone between the mandibular rami. These bones are also found in some extinct Ganoidei, and must not be confounded with the branchiostegal rays of Amia and Teleostei, which carry on the series of opercular bones ventrally.

1 This palatal cartilage has an upper portion of the hyoid fused with it in the embryo of Selachoidei.

The hyoidean and branchial arches are unossified in Elasmobranchii, Holocephali, Ceratodus, and Protopterus. The hyoid is an unsegmented rod in the Dog-fish (ScyIlium) and some other Sharks. It and the branchial arches are as a rule segmented and ossified typically (p. 93). The last arch present is usually rudimentary. There are seven branchial arches in Heptanchus, six in Hexanchus and Protopterus, five in other Fish. The fifth pair fuse into a median bone in Pharyngognathi (Teleostei). Gill-rakers or pointed cartilaginous processes crossing the branchial slits are borne upon the branchial arches in many Elasmobranchii, some Teleostei and the Dipnoi. Branchial rays radiate outwards from the branchial arches into the septa between the gill-pouches in Elasmobranchii: and in Sharks curved extra-branchial cartilages are attached to the bases of more or fewer of the branchial arches and lie close beneath the outer edges of the septa just mentioned.

The backbone is formed by the notochord with its sheath in a few Elasmobranchii (e. g. Echinorhinus), in Holocephali, chondrostean Ganoidei, Dipnoi. Bony rings, more numerous than the arches, are formed in the sheath in Holocephali. In other Fish there are ossified amphicoelous vertebrae, between which the notochord persists. Lepidosteus alone has opisthocoelous vertebrae, and in development an inter-vertebral thickening of the cartilage which divides, forming, as in Sauropsida and Amphibia, the opposing faces of adjoining vertebrae. The vertebral centra originate in Elasmobranchii by the growth of the bases of both neural and haemal arches round the notochordal sheath, and the fusion of the growths with the vertebral thickenings of the sheath. Ossification in the centra thus formed takes place concentrically (Cyclospondyli) or radially (Asterospondyli). The centra in bony Ganoidei and Teleostei are formed chiefly by parosteal ossification, imbedding the bases of the arches neural and haemal (p. 100). The centra are united at their edges by ligament.

The neural ridges form a complete investment to the spinal cord in Elasmobrcmchii and Holo-cephali. They are differentiated into neural arches ( = crural cartilages) which may or may not bear neural spines, and into intervening inter-crural cartilages which are placed intervertebrally. These parts may be incompletely ossified. The haemal ridges in the caudal region are differentiated similarly. Small intercrural cartilages are present in Aci-penser, and are perhaps represented in Lepidosteus and some Teleostei (p. 101). The ossified neural and haemal arches of Amia are connected to the vertebral centra by cartilage discs: in Polypterus, Lepidosteus, and Teleostei they are continuous with the centra and are formed by parosteal ossification round cartilage, which may or may not persist. The right and left arches are connected inter se above the spinal cord by either bone, cartilage, or ligament. The summits of the neural spines and of the haemal spines perhaps form the supports for the dorsal and anal fins respectively (p. 101). In all Fish a superior longitudinal ligament connects all the neural arches above the neural canal.

Bony zygapophyses are formed near the bases of the neural arches in some Teleostei. A cartilaginous rod terminates the backbone in Ganoidei, some Teleostei, and the Dipnoi. It is surrounded by a bony sheath (urostyle) in some Teleostei, and gives support to more than one neural and haemal arch.

There are only two regions in the backbone, a dorsal and a caudal - the latter distinguished by the ventral fusion of the haemal arches or ribs (p. 100). The largest number of vertebrae is found in Elasmobranchii. In some of the latter, e.g. ScyIlium and Raja, there are twice as many centra in the tail as there are myomeres and spinal nerves. The most anterior vertebrae coalesce with the skull in Ganoidei, some Teleostei and Dipnoi. In some Elasmobranchii at least, the two structures are articulated by distinct exoccipital condyles.

The ribs are cartilaginous in Elasmobranchii, Holocephali, chondrostean Ganoidei, and Dipnoi. They are always simple in form, and lie in Elasmobranchii in the fibrous septum between the dorso- and ventro-lateral muscles. In other Fish they lie close to the peritoneum, but their ends turn outwards in-Lepidosteus into the septum named. They correspond to the myocommata, in which slender bones (epi-centrals, epi-neurals, epi-pleurals) may be formed in some Teleostei.

As to the limbs, the ventral fins are absent in a few Teleostei, e.g. Muraenidae, Gymnotidae, among Physostomi; Syngnathidae among Lopho-branchii; Ostraciontidae and Gymnodonts among Plectognathi, or else represented by moveable spines, as in Balistidae and Triacanthidae in the order last named. Their occasional absence has been noted in Protopterus.

The shoulder-girdle is cartilaginous in Elasmobranchii, Holocephali, chondrostean Ganoidei and Dipnoi, ossified as a separate scapula and coracoid (claviculo-coracoid) in bony Ganoidei and Teleostei. It is of large size in the two first-named orders, and the right and left parts are either continuous ventrally or united by ligament. A distinct supra-scapula is present in many Elasmobranehii, especially in Rays, where it is attached to the vertebral column, and in the Sturgeon, which has also a large clavicular process. The form of the parts varies much, e.g. in Rays the articular region is of great extent and much fenestrated. The bones are small in Teleostei, and there are many variations in their mode of ossification. The Ganoidei and Teleostei have investing bones known as supra-clavicle, clavicle, inter-clavicle, and post-clavicle, all derived apparently from the skin and lining membrane of the branchial cavity, and present in none of the higher Vertebrata (p. 97); and a supra-temporal scale-bone (the first of the lateral line) connects the clavicular series to the epi- and opisth-otic regions of the cranium.