The family of _Chondrosteidæ_ includes the Triassic precursors of the sturgeons. The general form is that of the sturgeon, but the body is scaleless except on the upper caudal lobe, and there are no plates on the median line of the skull. The opercle and subopercle are present, the jaws are toothless, and there are a few well-developed caudal rays. The caudal has large fulcra. The single well-known species of this group, _Chondrosteus acipenseroides_, is found in the Triassic rocks of England and reaches a length of about three feet. It much resembles a modern sturgeon, though differing in several technical respects. _Chondrosteus pachyurus_ is based on the tail of a species of much larger size and _Gyrosteus mirabilis_, also of the English Triassic, is known from fragments of fishes which must have been 18 to 20 feet in length. [Illustration: FIG. 5.—_Chondrosteus acipenseroides_ Egerton. Family _Chondrosteidæ_. (After Woodward.) ] The sturgeons constitute the recent family of _Acipenseridæ_, characterized by the prolonged snout and toothless jaws and the presence of four barbels below the snout. In the _Acipenseridæ_ there are no branchiostegals and a median series of plates is present on the head. The body is armed with five rows of large bony bucklers,—each often with a hooked spine, sharpest in the young. Besides these, rhombic plates are developed on the tail, besides large fulcra. The sturgeons are the youngest of the Ganoids, not occurring before the Lower Eocene, one species, _Acipenser toliapicus_ occurring in the London clay. About thirty living species of sturgeon are known, referred to three genera: _Acipenser_, found throughout the Northern Hemisphere, _Scaphirhynchus_, in the Mississippi Valley, and _Kessleria_ (later called _Pseudoscaphirhynchus_), in Central Asia alone. Most of the species belong to the genus _Acipenser_, which abounds in all the rivers and seas in which salmon are found. Some of the smaller species spend their lives in the rivers, ascending smaller streams to spawn. Other sturgeons are marine, ascending fresh waters only for a moderate distance in the spawning season. They range in length from 2½ to 30 feet. All are used as food, although the flesh is rather coarse and beefy. From their large size and abundance they possess great economic value. The eggs of some species are prepared as caviar. [Illustration: FIG. 6.—Common Sturgeon, _Acipenser sturio_ Mitchill. Potomac River. ]