MR. GRAY ON TESTACEOUS MOLLUSCA. 305 



to the discrimination of the two genera, that M. Rang, looking only to the characters 

 of the shell, has proposed to reunite them into one. In proof of the little attention 

 that has hitherto been paid to this very important part, I may mention that three 

 species referred by Lamarck to the genus Solarium are each furnished with a different 

 kind of operculum ; and it is deserving of notice that the Monodonta canaliculata, 

 according to the observations of M. Quoy, has an operculum very different from the 

 rest of the shells of that genus. 



In some shells, again, the differences in character are so slight as almost to throw 

 an air of ridicule on the attempt to separate them generically from the structure of 

 the shells alone ; and yet when the animal is examined the necessity of their sepa- 

 ration becomes so obvious as to be immediately acknowledged. This is especially 

 the case with my genus Bullia compared with Terehra : the shells of these two ge- 

 nera are so similar, that Lamarck and all other conchologists have retained them in 

 one group, no other distinction being observable except that in the former there is a 

 more or less distinct callous band winding round the volutions just above the suture, 

 and produced by a slight extension of the inner lip beyond the part of the shell oc- 

 cupied by the whorl. This extension of the lip is probably deposited by the foot of 

 the animal, which in the genus Bullia is very large and expanded, while that of Te- 

 rehra is small and compressed. This, however, is not the only difference between the 

 two animals, that of the former genus having rather large and eyeless tentacles, while 

 the Terehrce have very small and short tentacles, bearing the eyes near their tips. 



A second example of a similar kind is derived from the genus Rostellaria, in which 

 Lamarck includes the Stromhus Pes Pelecani of Linnaeus. The animal of this shell 

 has been figured by Muller, and very much resembles that of Buccinum, having 

 long slender tentacles with the eyes sessile on the outer side of their base ; while, as 

 Dr. RuppELL informs me, the Rostellaria curvirostris has an animal allied to Stromhus, 

 with the eyes on very large peduncles, which give off from the middle of one of their 

 sides the small tentacles. Notwithstanding this difference in the form of their ani- 

 mals, 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 distin- 

 guished from the other Rostellarice. 



With all this uncertainty with regard to the generic characters of the recent spe- 

 cies of shells, of which the animals can be subjected to examination, how much must 

 the difficulty of deciding their genera with certainty be enhanced with reference to 

 the fossil species, and especially to those which have no strictly analogous form ex- 

 isting in the recent state. Considerations like these tend greatly to disturb the con- 

 fidence formerly reposed in the 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 geologist, is included under my second head, to the elucidation of which I 



shall now proceed. 



2 r2 



