PIKE, CEPLIALASPTS, POLYPTERUS, PrEiticiernys, and STURGEON, ante.
Order 5, Elasmobranekii.—This order is equivalent to the mlachia of Muller, the plaeoids of Agassiz, and the liolocephali and plagiostomi of Owen, and to the most typical portion of the cartilaginous fishes of Cuvier. It may be briefly defined by the following characters: The skull and lower jaw are well developed, but there are no distinct cra nial bones, the skull consisting of a single cartilaginous box, without sutures; vertebral column sometimes composed of distinct vertebra;, sometimes cartilaginous. The exo skeleton is composed of placoid granules, tubercles, or spines; two pairs of lateral fins, supported by cartilaginous fin rays. the ventral.fins far back; pectoral arch has no clavi cle; heart consists of one auricle and one ventricle, and a Mints artemosus rhythmically contractile and having a coat of striated muscular fibers, and several transverse rows of valves. The gills are pouch-like, differing materially from those in teleostei and gan oidei. In the sharks and rays the branchial arches are fixed, and the branehial cham ber is divided into a number of distinct pouches, and each partition supports a series of branchial laminae attached to each side like the leaves of a hook. The separate pouches open into the pharynx by a separate slit, and communicate externally with the water by a separate aperture placed on each side of the neck. There is, therefore, no gill-cover and no branchiostegal membrane or rays. The elasmobranchii are divided into two sub orders: Iwlieeplucli, in which the mouth is terminal, and there is only a single gill-slit, and the plagiostomi, in which the mouth is transverse and placed on the under surface of the head, and there are several gill-slits on each side of the neck. The suborder hobo cepliali includes several curious fishes, of which the only living forms are the chimeri dte. See CRULERA, ante. There are several fossil representatives in the secondary and tertiary formations constituting the genera edaphodus, elasmodus, and isehiodus. The suborder plagiostomi includes the sharks (q.v.) and the rays (q.v.).
Order 6, Dipnoi.—This order includes only certain kinds of mud-fishes, lepidosiren
and ceratodus, and is considered by some to be a connecting link between the fishes and the amphibia. See LEPIDOSIREN, ante. The lepidosiren formerly constituted the lowest order of amphibia, but most authorities now place it as the highest order among the fishes. The body is fishlike in form, the skull has distinct cranial bones and a lower jaw, and the notochord is persistent; there are no vertebral centra nor an occipital condyle. The axo-skeleton in living types is composed of overlapping cycloid scales, but the fossil forms have ganoid scales. The dipnoi resemble the amphibians in having a heart with three cavities—two auricles and one ventricle—and in having true lungs and a trachea and glottis, the blood returning to the heart by a distinct vein.
Distribution of Fiakes in Time.—The oldest representatives of fishes are. ganoids, and four genera are found in the upper Silurian rocks. Their maximum development was in the Devonian red sandstone. The stureouidm commenced their existence in the mesozoic formations (lias) and a true sturgeon, ampeaser toliapieus, in the eocene. The elasmobranchii (sharks and rays) commenced at the close of the upper silnrian in this upper Ludlow rocks. Their remains consist of spines and fin rays, from which the genius oneitas has been established. Portions of shagreen have been referred to the genus itpltagodus. The true sharks are represented in the later mesozoic deposits. but they are mostly found in the tertiary formation, several genera in the eocene. The true rays, however, arc older than the true sharks, probably beginning in the carboniferous rocks. Until recently the dipnoi were not known to be represented in past time, but the trias sic and Jurassic teeth upon which Agassiz founded his genus eeratod use are now regarded as belonging to this order. The teleost fishes first made their appearance in the creta ceous formation, toward the end of the mesozoic epoch, but after this they are the chief representatives of the whole class of fishes. and they now seem to have reached their greatest development.