200 FISHES. 



vertebrates; in a few Ganoids the air bladder is cellular, 

 and more or less functional and connected by a glottis 

 with the oesophagus; in most of the soft -rayed Teleo- 

 cephdli there is a slender duct connecting the air bladder 

 with the alimentary canal; in the Acanthopteri and others 

 this is wanting. Reproduction by eggs of small size, 

 which are fertilized generally after exclusion; a few are 

 ovoviviparous. 



As here characterized, the class Pisces includes the 

 Teliosts and Ganoids, of authors, and excludes the 

 Sharks and Skates and their allies, as well as the Lam- 

 preys and Lancelets, which differ from the true Fishes 

 more than the latter do from the Batrachians. 



The following key includes not only the families of 

 fresh water fishes described in this work, but also all of 

 the families of Fishes represented on the Atlantic Coast 

 of the U. S. The names of those families which are ex- 

 clusively marine are printed in italics, and no further 

 reference is made to them. A student, therefore, who 

 traces a fresh water fish to any of them will understand 

 that there is an error on his part or mine. The key is, 

 of course, purely artificial, and does not, in most cases, 

 give true family distinctions, for instance: 



With 5 to 9 detached finlets behind dorsal and anal ; dorsals 2 ; 

 scales small or none. . . Scombridcv, the. Mackerels. 



does not imply that all Scombridw possess those charac- 

 ters, nor, indeed, that all possessing them are tScombridce; 

 but that all fishes in the region here covered, which show 

 those peculiarities, are to be referred to that family. 



SUB-CLASS I. Tail homocercal (caudal fin rarely wanting) ; optic 

 nerves simply crossing, without chiasma ; arterial bulb simple, 

 with two opposite valves at its origin; air bladder, if present, 

 not cellular; exoskeleton typically of scales, either ctenoid or 

 cycloid TELEOSTEI, page 201. 



