TURBIDJE. 3 



which is founded, as most are, upon a partial 

 view of the organization. Thus the lowest 

 members of the order Pisces, as the genera 

 Amphioxus and Gastrobranchus, although they 

 belong to one of the vertebrate classes, are 

 lower in the scale of being than the insecta, 

 which, in their turn, have been improperly 

 placed below the akephalous mollusca, because 

 the latter have a heart, an organ which could 

 not be readily demonstrated in the former. 



Belonging to a family or division the greater 

 part of the members of which are marine, I 

 will not attempt to take a general view of the 

 whole, contenting myself with giving the history 

 of each of the four fluviatile genera found in 

 the United States, under its proper head. 



In the arrangement of genera, as far as this 

 work is concerned, (it having been necessary 

 to determine the binder's signatures from the 

 first,) it was deemed best to place the Melaniadae 

 at the head, and enter the present family by 

 Anculosa and Amnicola, which are nearly 

 allied; leaving the Physadae by Ancylus to pass 

 to the Limacidse. 



