The Teleostei form the majority of the fishes of the present fauna, and are the geological successors of the Palæichthyes, undoubted Teleostei not ranging farther back than the Chalk. This sub-class comprises an infinite variety of forms; and as, naturally, many Ganoid fishes lived under similar external conditions, and led a similar mode of life as certain Teleostei, we find not a few analogous forms in both series: some Ganoids resembling externally the Teleosteous Siluroids, others the Clupeoids, others the Chætodonts, others the Scombresoces, etc. But there is no direct genetic relation between those fishes, as some Naturalists were inclined to believe.
The Teleostei are divided into six orders:—
A. Acanthopterygii.—Part of the rays of the dorsal, anal, and ventral fins not articulated, spines. The lower pharyngeals separate. Air-bladder, if present, without pneumatic duct in the adult.
B. Acanthopterygii pharyngognathi.—Part of the rays of the dorsal, anal, and ventral fins not articulated, spines. The lower pharyngeals coalesced. Air-bladder without pneumatic duct.
C. Anacanthini.—Vertical and ventral fins without spinous rays. Ventral fins, if present, jugular or thoracic. Air-bladder, if present, without pneumatic duct. Lower pharyngeals separate.
D. Physostomi.—All the fin rays articulated; only the first of the dorsal and pectoral fins is sometimes ossified. Ventral fins, if present, abdominal, without spine. Air-bladder, if present, with a pneumatic duct.
E. Lophobranchii.—Gills not laminated, but composed of small rounded lobes, attached to the branchial arches. Gill-cover reduced to a large simple plate. A dermal skeleton replaces more or less soft integuments.
F. Plectognathi.—A soft dorsal fin opposite to the anal; sometimes elements of a spinous dorsal. Ventral fins none, or reduced to spines. Gills pectinate; air-bladder without pneumatic duct. Skin with rough scutes, or with spines, or naked.
FIRST ORDER—ACANTHOPTERYGII.
Part of the rays of the dorsal, anal, and ventral fins are not articulated, more or less pungent spines. The lower pharyngeals are generally separate. Air-bladder, if present, without pneumatic duct in the adult.[41]