SIMILAR SHELLS. 407 



alone ; and yet when the animal is examined the necessity 

 of their separation becomes so obvious as to be immediately 

 acknowledged. This is especially the case with my genus 

 Bullia compared with Terebra : the shells of these two 

 genera are so similar, that Lamarck and all other concholo- 

 gists 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 occupied 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 Terebra 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 Terebras 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 Strombus 

 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. Riippell 

 informs me, the Rostellaria curvirostris has an animal allied 

 to Strombus, 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 animals, I am not, however, aware of any essential 

 character by which the shell of Aporrhais (as the Strombus 

 pes-pelecani has been generically named) can be distin- 

 guished from the other Rostellariae. 



" With all this uncertainty with regard to the generic 

 characters of the recent species 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 existing in 

 the recent state. Considerations like these tend greatly to 

 disturb the confidence 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." 



