658 



APPENDIX B 



of skin. The fins are paired. The skin has imbedded denticles or "plaeoid scales" 

 that have the general structure of a tooth. The heart is two chambered. 



There are two subclasses, of which the Elasmobranchii is by far the largest. 

 This group is known back to the Devonian, and includes the extinct *Cladose- 

 lachii (Fig. 27.12) and the order Selachii. To the latter belong many extinct and 

 all modern elasmobranchs, including the sharks, skates, sawfishes, sting rays, or 



Fig. B.28. Sharks and rays, class Chondrichthyes. Above, an eagle ray, Aetobatus marinari. 

 Below, a group of sharks, including the white, hammerhead, southern ground, spot-fin 

 ground, and tiger sharks. Also shown is the marine loggerhead turtle. (Courtesy American 

 Museum of Natural History.) 



stingarees, electric rays, and the giant mantas (also called eagle rays or sea vam- 

 pires). To the second subclass, Holocephali, belong only the strange chimaeras. 



Class 4. The True or Bony Fishes (Osteichthyes). Cold-blooded aquatic verte- 

 brates with internal skeleton bony or partly bony; gills covered by a bony flap 

 (operculum); scales usually present, of "ganoid" or "cosmoid" type in primitive 

 forms, in modern teleost fishes thin and horny (cycloid or ctenoid), never of 

 "plaeoid" type. Paired fins usually present. Heart two-chambered, approaching 

 the three-chambered condition in Dipnoi. 



Subclass 1. The choanate fishes (Choanichthyes) have internal nostrils (choanae) 

 enabling them to breathe air without opening the mouth. This subclass includes 

 the lungfishes (Dipnoi, Fig. 27.16), with fan-shaped tooth platen, without maxil- 



