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XVI. Remarks on the difficulty of distinguishing certain Genera of Testaceous 

 Mollusca by their Shells alone, and on the Anomalies in regard to Habitation 

 observed in certain Species. By John Edward Gray, Esq. F.R.S. S^c. 



Received June 11, — Read June 18, 1835. 



It has been a very common error, both among conchologists and geologists, to re- 

 gard all shells in which no remarkable difference of form and character can be distin- 

 guished as inhabited by one and the same genus of animals ; and not less usual to 

 assume that all the species of the same genus inhabit similar localities. Many geo- 

 logists have still further enlarged the boundaries of error, by taking for granted that 

 all the fossil species of shells which are referrible by the characters of the shell to 

 recent genera, must have been formed by animals which, in their recent state, pos- 

 sessed the same habits as the most commonly observed species of the genus to which 

 they appear to belong. These theories were, indeed, quite consistent with our former 

 ignorance of the habits of the animals of this class ; but since the works of Poli, 

 MiJLLER, Montagu, Lamarck, and Cuvier have induced zoologists again to turn 

 their attention, as was the practice among the older writers, to the animals of shells, 

 and their habits, and no longer to confine themselves, as was too often the case with 

 the followers of the Linnean system of conchology, to the study of the shells as mere 

 pieces of ornament, classed without reference to their inhabitants, the acknowledged 

 importance of the subject is daily bringing to our knowledge some animal unknown 

 before, and adding to our stock of information facts which prove the fallacy of the 

 opinions so hastily taken up. Thus, although even at the present day the animals of 

 less than one twentieth part of the well-known species of shells have been observed, 

 — and of those which are known the greater part have been very imperfectly de- 

 scribed, — numerous exceptions to the theories in question have been brought to 

 light, which deserve to be collected into one point of view, and made the subject of 

 serious consideration. 



The exceptions which it is the object of the present paper to notice may be ar- 

 ranged under the two following heads : 



1. Shells having every appearance of belonging to the same natural genus, but in- 

 habited by animals of a very different character. 



2. Species of testaceous Mollusca living in very different situations from the majo- 

 rity of the known species of the genus to which they belong, or having the faculty of 

 maintaining their existence in several different situations. 



These two classes of exceptions I shall proceed to notice in detail. 



