136 ON THE NAIADES 



of knowledge, while there is such a " diffusion of existing knowledge 

 among the mass of mankind" as we have at present. 



We can only account for the almost total neglect of the family 

 Naiades by writers on natural history of the last century, in the fact 

 that the fresh waters of Europe produce so few species that they had 

 then scarcely attracted attention. The habits of these animals have 

 been there so little studied and known, that some recent writers of 

 reputation assert that they move with the beaks of the shell " down- 

 ward," which is equivalent to saying they walk on their backs. The 

 anterior part has been called the posterior part, which is as much as 

 to say, that their locomotion is backward. 



These facts display a great want of attention to the animal in its 

 element, — where it would be observed to possess many curious and 

 striking characteristics. The great systematist, the immortal Linneus, 

 whose name will be found recorded in the book of the last student of 

 natural history, knew so few members of this family that he classed 

 them indiscriminately with two marine genera, Mya and Mytilus. 



It was the rich and splendid productions of the rivers of the United 

 States, and particularly those which are tributary to the Mississippi, 

 which first roused the attention of the zoologist to their extraordinary 

 characters ; and they have within a few years become sought after by 

 collectors as eagerly as the " most precious jewels of the ocean." 



Urged by the solicitations of numerous scientific friends, I have 

 continued my efforts to obtain such specimens as appeared to me to be 

 new and undescribed, and they are now submitted to the consideration 

 of the Society. 



In my communications I have heretofore said little on the geogra- 

 phical position of our Naiades. It has, however, been to me an inte- 

 resting branch of the subject, and engaged much of my attention. 

 The great dividing ridge or chain of mountains, the AUeghanies, which 

 seems so completely to separate our eastern from our western waters, 

 almost as completely separates the species of this family inhabiting 

 those parts lying east and west of it. It is a matter of doubt if there 

 be more than two or three species of all the genera of this family ex- 

 isting in the eastern waters which have their analogues in the western 

 waters. That shell, which we have considered the Unio cariosus of 



