132 



VERTEBRATE ZOOLOGY 



or Spoon-Bill (Polyodon), and the sturgeon (Acipenser). These are 

 considered to represent the culmination of degenerative evolutionary 

 processes, combined with certain marked specializations. 



The Spoon-Bill (Fig. 71, B) inhabits the Mississippi River and 

 its tributaries. It is a sluggish creature, feeding chiefly on mud that 

 it shovels up with its spade-like snout. The mud is strained through 

 gills provided with unusually long gill-rakers, which serve to catch the 

 food particles and let the mud go through with the water current. 

 The paddle-like rostrum is richly supplied with tactile end-organs, 



FIG. 71. Chondrostei. A. The Sturgeon, Acipenser ruthenus (after Cuvier). 

 B. The Spoon-Bill or Paddle-Fish, Polyodon folium (after Bridge). 



enabling the fish to detect the presence of food in the mud. An al- 

 lied genus of Spoon-Bill (Psephurus) inhabits some of the principal 

 rivers of China and evidently lives the same type of life as Polyodon. 

 The shape of the body is decidedly selachoid or shark-like, and the 

 skin is apparently scaleless, though scattered vestigial scales are 

 found, and a patch of rhombic scales occurs on the upper hah of 

 the caudal lobe. 



The Sturgeon family (Fig. 71, A) is a widely distributed group, 

 superficially decidedly selachoid in appearance. The rostrum is pro- 

 longed into a snout with a transverse row of barbels depending from 

 its ventral surface. The mouth is small and protrusible, but is with- 



