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Cambridge, 26th July, 1851. 

 My Dear Sir, — I ought long ago to have answered your letter re- 

 specting the Naiades, but pressing engagements have made it an im- 

 possibility for me to attend to my correspondence for some time past. 

 Moreover, I did not fidly understand the drift of your claim, and con- 

 scious of duly appreciating the real value of your investigations in 

 the history of that highly interesting group of molliisks, it did not 

 occur to me that you could suspect me of even an accidental neglect 

 of your observations. There cannot be a single naturalist at all con- 

 versant with that subject, who has not read and studied your nume- 

 rous and highly valuable papers on the fresh water and land shells, as 

 well as on the fossils. But I find from the proceedings of the Ameri- 

 can Philosophical Society, that there is such an apprehension on your 

 pai't. Let me therefore say, what you might have already suspected, 

 even from the very short abstract of my communication published in 

 the proceedings of the Boston Natural History Society, that my ob- 

 ject in investigating the structure of Naiades has been very different 

 from yours. If you will await the publication of my paper, you will 

 satisfy yourself that I know exactly who has first observed this or 

 that fact respecting the organization of these animals, and that I give 

 every body due credit for what he has done. But I have been testing 

 the peculiarities of structure of the Naiades with the view of finding 

 new characters for classifying, or rather dividing them into genera. 

 Whether the facts alluded to were known or not, is a matter of no 

 consequence in this point of view, though many of the points alluded 

 to by me have not been noticed before, as you will perceive in read- 

 ing my paper which is soon to appear ; and if I use as generic cha- 

 racters, in dividing the Naiades into many genera, as you will find I 

 do, peculiarities which have never been used as such, you will, no 

 doubt, acknowledge that I have introduced new characters in the 

 classification of these animals, and, as I believe, greatly improved 

 their natural arrangement. You will, indeed, find that 1 use as ge- 

 neric characters, peculiarities which you have considered as having 

 no other importance than (hat of affording additional facilities for the 

 distinction of species; characters which, in your opinion, cannot even 

 be employed as distinguishing natural groups, since even in your 

 latest synopsis they are nowhere introduced as such. You will see 

 moreover, that, whether I am right or wrong is another question, I 

 have been led by the use of those very characters, in most instances, 

 to separate from one another species which you have united as form- 

 ing natural divisions, and to bring together species which you have 

 VOL. V. — 2 H 



