STURGEON TRIBE. 
Toothed Tile 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 (^g- nat. 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. 1 he 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 lows of scales 
vol. v.—33 
