The Chondropterygii are subdivided into three orders:
(a) Sturionidæ (sturgeons), with free gills.
(b) Selacii (rays, sharks), with gills fixed and a mouth formed for mastication.
(c) Cyclostomata (lamprey, myxine), with gills fixed and a mouth formed for suction.
The osseous fishes, which are far more numerous, are subdivided into six orders:
(a) Acanthopterygii; distinguished by the stiff spines which constitute the first fin-rays of the dorsal fin, or which support the anterior fin of the back in case there are two dorsals. In some cases the anterior dorsal fin is only represented by detached spines. The first rays of the anal fin are likewise spinous, as well as the first ray of the ventral fin. To this extensive order, which comprises about three-fourths of the osseous fishes, belong, among others, the families of the perches, gurnards, mackerels, mullets, breams, gobies, blennies, &c.
The three following orders of the osseous fishes have the rays that support the fins soft and composed of numerous pieces articulated with each other, with the exception in some cases of the first ray of the dorsal, or of the pectoral. Their leading character is afforded by the situation or absence of the ventral fin, which in the
(b) Malacopterygii abdominales are suspended beneath the abdomen, and behind the pectorals; in the
(c) Malacopterygii subbrachiales beneath the pectorals; and in the
(d) Malacopterygii apodes are totally wanting.