THE STURGEON TRIBE 



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former division the pelvic fins have their superior row of supporting ossicles, or 

 baseosts, rudimental or wanting, in the present group these are well developed. 

 The living representatives of the sturgeon tribe agree with the bow fish and its 

 allies in the want of any interlacing of the fibres of the optic nerves at their cross- 

 ing, and likewise in the presence of a spiral valve to the intestine. In both the 

 living and extinct types the tail is of either the diphycercal or heterocercal type. 

 As a suborder, the sturgeon tribe may be characterized by the more or less com- 

 pletely persistent notochord, by the inferior and superior supporting ossicles 

 (axonosts and baseosts) of the dorsal and anal fins forming a simple and regular 

 series, and also by the presence of a pair of infraclavicular plates in the pectoral 

 girdle. In all the known forms there is a single dorsal and anal fin, both of which 



SPOON-BEAKED STURGEON. 



(One-fifteenth natural size.) 



are well separated from the caudal; while in the existing members the air bladder 

 is furnished with a duct. Although represented at the present solely by the stur- 

 geons and their allies, the group was very abundant during the Secondary epoch; 

 and whereas the sturgeons, together with certain extinct families, form what may 

 be termed a degenerate specialized series characterized by the absence of ganoid 

 scales in a second and normal series the body was covered with such scales. 



The toothed sturgeons, of which there are two existing representa- 

 tives, each forming a genus by itself, constitute the family Polyodon- 

 tides. 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 





