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AN APPENDIX TO LETTER XVI. 



OF SPECIES BELONGING TO THE SAME NATURAL GENUS 



INHABITING ESSENTIALLY DIFFERENT SITUATIONS. 



BY JOHN EDWARD GRAY, ESQ., F.R.S. &C. 



Reprinted from the Philosophical Transactions, part ii. for 1835, with 

 the consent of the Author. 



THE general belief that all the species of the same genus 

 inhabit the same kind of situation, undoubtedly holds good 

 with reference to most of the genera of shells ; but many 

 exceptions have already been observed, and we may anti- 

 cipate that many more will be discovered as the natural 

 habits of the different species become better known. In 

 bringing together a number of these exceptions, I have been 

 under the necessity of placing considerable reliance on the 

 observations of others, who have noted in foreign countries 

 facts similar to those which I have myself witnessed at 

 home ; but these observations have been chiefly collected 

 from the works of Professor Nilsson of Sweden, of Mr. Say 

 of the United States of North America, and of MM. Lesson, 

 Quoy, and Rang of Paris, writers who, from their extensive 

 knowledge of Conchology, are fully capable of accurately 

 recording their observations, and whose statements may 

 therefore be received as deserving of the most implicit con- 

 fidence. It is moreover to be observed, that all their ob- 

 servations on this subject were made simply with the view 

 of extending the knowledge of the history of the species to 

 which they refer, and without reference to the establishment 

 of any preconceived theory. 



These observations may be classed under the four follow- 

 ing subdivisions : 1st, where species of the same genus are 

 found in more than one kind of situation, as on land, in 

 fresh and in salt water ; 2nd, where one or more species of 

 a genus, most of whose species inhabit fresh water, are found 

 in salt or brackish water ; 3rd, where, on the contrary, one 

 or more species of a genus, whose species generally inhabit 

 the sea, are found in fresh water ; and 4th, where the same 

 species is found both in salt and fresh water. 



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