BONY FISHES AND GANOIDS. 511 



type. It might be thought that the bony shields investing the head, and the 

 longitudinal rows of prominent bony tubercles or plates frequently present 

 on the otherwise entirely naked skin, would form a distinctive feature of the 

 group. As a matter of fact this is not so, seeing that a number of extinct 

 tishes now classed with the sturgeons have a complete coat of ganoid scales. 

 Still, however, as distinctive of the existing forms — with which alone we are 

 concerned here — the presence of these shields or plates, or both, coupled with 

 the absence of scales (save in one family on part of the tail) are very dis- 

 tinctive. Existing sturgeons may be classified in two families, in both of 

 which the tail is heterocercal. 



The first of those is represented only by two genera, each with a single 

 species, and its distinctive features are to be found in the presence of minute 

 teeth in the jaws throughout-life, as well as of a median series 

 of unpaired bony shields in the armour of the head, and Family 



likewise by the skin — save for a few minute stellate ossifica- Polyodonlidce. 

 tions — being naked all over the body, although there may be 

 a few scales on the upper lobe of the tail. The typical Folyodon folins, from 

 the Mississippi basin, is a comparatively small fish, apparently not exceeding 

 about six feet in length, characterised by the upper jaw terminating in an 

 enormous shovel-like beak, furnished with soft thin margins, and equal to 

 fully one quarter the entire length of the fish. Vastly larger is Psephurus 

 gladius from some of the great Chinese rivers, which grows to something 

 approaching twenty feet, and has a much more slender beak. Both these 

 fishes have minute eyes, and appear to grovel in the mud in the river bottom 

 in search of their food. The occvirrence of two such closely allied forms in 

 the rivers of North America and China is paralleled by the instance of the 

 American and Chinese alligators, and likewise the two kinds of giant 

 salamander (Megalobatrachus and Cryptobranchv.s), all these examples indicat- 

 ing the close affinity between the fauna of North America and that of North- 

 Eastern Asia. 



The members of the second family of the group differ from the Pohjodon- 

 tidce by the disappearance of the teeth in the adult, and likewise by the bare 

 skin of the body being traversed by five longitudinal rows of 

 large elevated bony plates. The median series of shields is Family 



also wanting on the head, and the under surface of the Acipenseridai. 

 muzzle is provided with two pairs of flexible barbels, of 

 which there is no trace in the first family. All the members of the family 

 are restricted to the temperate parts of the Northern Hemisphere, and all 

 are largely fresh water in their habits, some being exclusively so, although 

 others ascend the larger rivers 

 only for the purpose of spawn- 

 ing, after which they return to 

 the ocean. Whereas the typical 

 members of the family belong 

 to the genus Acipenser, certain 

 fresh-water kinds from the basin 

 of the Mississippi and Central Figi. 34.— Common Stukoeon. 



Asia form a genus apart, under 



the name of Scaphirhynchitit, one of their distinctive features being the pro- 

 duction of the muzzle into a shovel-like beak very similar to that of Psephurus. 

 Of the typical genus, the largest species is the huso (Acipenser huso) of the 

 Russian rivers and inland seas, which occasionally attains the enormous 



