80 Mr Gray on Testaceous Mollusca, 



assume that all the species of the same genus inhabit similar 

 localities. Many geologists have still further enlarged the bound- 

 aries of error, by taking for granted that all the fossil species of 

 shells which are referable by the characters of the shell to re- 

 cent genera, must have been formed by animals which, in their 

 recent state, possessed 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, Mliller, Montagu, Lamarck, and Cuvier have 

 induced zoologists again to turn their attention, as was the prac- 

 tice 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 with- 

 out 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 described — 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 arranged under the two following heads : — 



1. Shells having every appearance of belonging to the same 

 natural genus, but inhabited by animals of a very different cha- 

 racter. 



2. Species of testaceous Mollusca living in very different si- 

 tuations from the majority 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. 



