INTRODUCTION. 



As the aqueous surface of the globe is sup- 

 posed to exceed the terrestrial by one third, 

 it is evident that the inhabitants of the seas 

 and rivers must form a very distinguished 

 feature in Zoology. In the vast world of 

 waters the animal kingdom presents the ex- 

 tremes of bulk and minuteness ; from the 

 myriads of minute beings, which would be 

 invisible to us without the assistance of the 

 microscope, up to those stupendous masses, 

 whales and cachalots. Until natural history 

 attained some precision, the term " fish," 

 however, was often misapplied, and was used 

 to include all aquatic animals whatever ; 

 thus cetaceous, molluscous, and crustaceous 

 animals were most erroneously admitted into 

 the class of Fishes. 



Fortunately for the necessary precision of 

 zoological classification, fish are now readily 

 separated from other vertebrated animals, by 

 their capability of absorbing atmospherical 



