86 Mr Gray wi Testaceous Mollusca, 



their sides the small tentacles. Notwithstanding this difference 

 in the form of their animals, I am not, however, aware of any 

 essential character by which the shell of Aporrhais (as the 

 Stromhus Pes Pelecani has been generically named) can be dis- 

 tinguished from the other Rostellarice. 



With all this uncertainty with regard to the generic charac- 

 ters of the recent species of shells, of which the animals can be 

 subjected to examination, how much must the difficulty of de- 

 ciding their genera with certainty be enhanced with reference to 

 the fossil species, and especially to those which have no strictly 

 analogous form existing in the recent state. Considerations like 

 these tend greatly to disturb the confidence formerly reposed in 

 die opinion, that every difference in the form and structure of 

 the animal was accompanied by marks permanently traced upon 

 the shell_, by which it might be at once distinguished, and which 

 it was therefore the great object of the conchologist to point out. 

 But another source of error, particularly interesting to the geo- 

 logist, is included under my second head, to the elucidation of 

 which I shall now proceed. 



2. Of Species belonging to the same natural Genus^ inhabiting essen- 

 tially different situations. 



The general belief that all the species of the same genus in- 

 habit 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 anticipate 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 ex- 

 tensive knowledge of conchology, are fully capable of accurately 

 recording their observations, and whose statements may there- 

 fore be received as deserving of the most implicit confidence. It 

 is moreover to be observed, that all their observations on this 



