STURGEON TRIBE. 



5 J 3 



Toothed The toothed sturgeons, of which there are two existing repre- 



sturgeons. sentatives, each forming a genus by itself, constitute the family 

 Polyodontidce. While agreeing with the other members of the series in having 

 the cartilaginous skull invested with a series of superficial bony plates, these fishes 

 are specially distinguished by possessing a median unpaired series of bones in this 

 shield ; by the absence of branchiostegal rays ; the presence of minute teeth in the 

 adult ; the heterocercal tail ; and by the skin being either naked or with some scales 

 on the upper lobe of the tail. The first of the two existing genera is represented 

 by the spoon-beaked sturgeon (Polyodon folius) of the Mississippi, which grows to a 

 length of 6 feet, and is characterised by the production of the upper jaw into a 

 very long spoon-like beak, with thin, flexible margins, equal to one-fourth the 



SPOON-BEAKED STURGEON (^ Hat. size). 



total length in the adult, but still longer in the young. The gill-cover ends in a 

 long tapering flap ; the upper lobe of the tail bears a numerous series of narrow 

 fulcra; and the air-bladder is cellular. On the other hand, the slender-beaked 

 sturgeon (Psephurus gladius) from the Yang-tse-kiang and Hoangho rivers of China, 

 differs in the more conical form of the beak, and in the large size and small number 

 of the caudal fulcra. Growing to an enormous length — it is said as much as 

 20 feet — this fish agrees with the preceding in the very small size of its eyes, from 

 which it may be inferred that both seek their prey without depending upon sight. 

 Indeed, in the muddy waters of the rivers they inhabit, eyes can be of little use, 

 and it has been suggested that these fish depend chiefly upon their beak, which is 

 probably employed as an organ of touch. The flesh of both species is eaten. 

 Among several fossil forms, we may mention the genus Crossopholis, of the North 

 American Eocene, on account of the retention of a series of oblique rows of scales 

 vol. v.— 33 



