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TEXTBOOK OF ZOOLOGY 



Subclass Teleostomi. — True Fishes. In this group are found all 

 fresh-water fishes and many of the marine fish which frequent our 

 waters and shores, except lamprey. In this general division of the 

 group are four orders in the world, each including subdivisions of 

 either families or in the larger orders, suborders and families. 



Order Crossopterygii (Family Polypteridae) . — Polypterus, an Afri- 

 can fish using the swim bladder as an accessory respiratory organ. 



Fig. 243. — Polydon spathula, spoonbill cat or paddle fish and its associates. (Cour- 

 tesy of American Museum of Natural History.) 



Order Chondrostei. — Paddlefishes and Sturgeons. A ganoid type 

 of fish with abundant proportion of cartilage in the skeleton and with 

 bony ganoid scales. 



Family Polyodontidae. — Paddlefishes or Spoonbills. Polydon spath- 

 ula is found in the Mississippi Valley. It has a smooth skin and 

 long, flat paddlelike snout, with a few ganoid scales on the tail. 



Family Acipenseridae. — Sturgeons. There are only three genera 

 usually described for this country. The body is covered with five 

 rows of keeled, ganoid shields and the tail is distinctly heterocercal 



