Sturgeons are, perhaps, the geologically youngest Ganoids, evidence of their existence not having been met with hitherto in formations of older date than the Eocene clay of Sheppey. They are exclusively inhabitants of the temperate zone of the Northern Hemisphere, being either entirely confined to fresh water, or passing, for the purpose of spawning, a part of the year in rivers. They grow to a large size, and are the largest fishes of the fresh waters of the Northern Hemisphere, specimens 10 feet long being of common occurrence. The ova are very small, and so numerous that one female has been calculated to produce about three millions at one season; therefore their propagation, as well as their growth, must be very rapid; and although in many rivers their number is annually considerably thinned by the systematic manner in which they are caught when they ascend the rivers in shoals from the sea, no diminution has been observed. Wherever they occur they prove to be most valuable on account of their wholesome flesh. In Russia, besides, two not unimportant articles of trade are obtained from them, viz. Caviare, which is prepared from their ovaries, and Isinglass, which is made from the inner coats of their air-bladder. True Sturgeons are divided into two genera, _Acipenser_ and _Scaphirhynchus_. ACIPENSER.--The rows of osseous bucklers are not confluent on the tail. Spiracles present. Caudal rays surrounding the extremity of the tail. About twenty different species of Sturgeons may be distinguished from European, Asiatic, and American rivers. The best known are the Sterlet (_A. ruthenus_) from Russian rivers, celebrated for the excellency of its flesh, but rarely exceeding a length of three feet; the Californian Short-snouted Sturgeon (_A. brachyrhynchus_); the Hausen (_A. huso_), from rivers, falling into the Black Sea and the Sea of Azow (rare in Mediterranean), sometimes 12 feet long, and yielding an inferior kind of isinglass; the Chinese Sturgeon (_A. sinensis_); the Common Sturgeon of the United States (_A. maculosus_), which sometimes crosses the Atlantic to the coasts of Great Britain; Güldenstædt’s Sturgeon (_A. güldenstædtii_), common in European and Asiatic rivers, which yields more than one-fourth of the caviare and isinglass exported from Russia; the Common Sturgeon of Western Europe (_A. sturio_), which attains to a length of 18 feet, and has established itself also on the coasts of Eastern North America. SCAPHIRHYNCHUS.--Snout spatulate; posterior part of the tail attenuated and depressed, so that it is entirely enveloped by the osseous scutes. Spiracles none. The caudal rays do not extend to the extremity of the tail, which terminates in a filament.