96 Molluscs of the isles Sechelles and Amirantes. 



ral. M. Dufo has confirmed, on a rather extensive range 

 of species of the genera (Fusus) Turbenella, Murex, 

 Purpura, Buccinum, &c. that this method of procedure was 

 well founded. Thus he has demonstrated by the oper- 

 culum of the pretended Buccinum undosum that it was a 

 species of the genus Turbinella, and by that of the Cerithium 

 paulustre, which differs by its composition of imbricated 

 circular elements from that of true Cerithia, that this shell 

 did not belong to this genus. And thus is confirmed the 

 genus Potamida, established by M. Alexander Brongniart 

 for the fossil shells considered before him as Cerithia, the 

 fluvatile nature of the earth in which we find them, as well 

 as certain peculiarities in the form of their opening, led to 

 their separation from the Cerithia, which are marine. In 

 effect the Cerithium palustre, as its name indicates, inhabits 

 fresh water marshes. 



Science owes also to M. Dufo the positive fact of the 

 absence of the operculum in the genus Terebellum, which be- 

 fore him was only suspected. 



The second point on which the observations of M. Dufo has 

 more essentially been directed, is that of the successive form 

 by which shells pass from the incipient existence of the crea- 

 ture which carries them to their deposition ; it is also a point 

 extremely important, and evidently in relation with the 

 fact of diminution of the lobes of the mantle with age, as M. 

 Dufo has again confirmed. 



Since indeed that geology, as it were struggling for a 

 scientific existence, admits in organized bodies the remains 

 of which exist in the superficial strata of the earth, one of 

 the elements the most calculated for the resolution of her pro- 

 blems of identity or antiquity, and even the etiology of strata, 

 the study of shells which by their chemical nature may con- 

 tribute to the formation of extensive rocks ; this study should 

 acquire, and indeed has acquired, great importance ; but un- 

 happily since M. Lamarck, so justly celebrated, has regulated 



