GANOIDEI. 361 



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, speci- 

 mens 10 feet long being of common occurrence. The ova are 

 very small, and so numerous that one female has been calcu- 

 lated 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 numlier is annually con- 

 siderably 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 

 fiesh. In Eussia, 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, Acipcnscr 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 dis- 

 tinguished from European, Asiatic, and American rivers. The 

 best known are the Sterlet {A. ruthenits) from Eussian rivers, 

 celebrated for the excellency of its flesh, but rarely exceeding 

 a length of three feet ; the Californian Short-snouted Sturgeon 

 (A. hrachyrhynchns) ; the Hansen [A. Juiso), from rivers, 

 falling into the Black Sea and the Sea of Azow (rare in 

 Mediterranean), sometimes 12 feet long, and yielding an in- 

 ferior kind of isinglass ; the Chinese Sturgeon (A. sinensis) ; 



