418 LOUIS AGASSIZ. 



and Jurassic deposits. . . . Imagine that all 

 this is at my full disposal for description and 

 illustration, and you will understand my pleas- 

 ure. The liberality of the American natural- 

 ists toward me is unparalleled. 



I must not omit to mention Mr. Lea's col- 

 lection of fresh- water shells, a series of the 

 magnificent Unios of the rivers and lakes of 

 America, comprising four hundred species, 

 represented by some thirty specimens of each. 

 Mr. Lea has promised me specimens of all 

 the species. Had I not been bound by an en- 

 gagement at Washington, and could I have 

 remained three or four days longer in order 

 to label and pack them, I might have taken 

 at once these valuable objects, which will be 

 of great importance in verifying and rectify- 

 ing the synonyms of European conchologists. 

 After having seen the astonishing variations 

 undergone by these shells in their growth, I 

 am satisfied that all which European natural- 

 ists have written on this subject must be re- 

 vised. Only with the help of a very full se- 

 ries of individuals can one fully understand 

 these animals, and we have only single speci- 

 mens in our collections. If I had time and 

 means to have drawings made of all these 

 forms, the collection of Mr. Lea would be at 



