Mr Gray on Testaceous Mollusca. 85 



A similar difficulty exists with respect to other Littorina as dis- 

 tinguished from Phasianella, and with the Neritince as distin- 

 guished from x\\eNeritcB. In the latter case the characters derived 

 from the operculum are so essential 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 at- 

 tention 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 oper- 

 culum ; and it is deserving of notice that the Monodonta cnna- 

 liculata according to the observations of M. Quoy, has an oper- 

 culum 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 separation 

 becomes so obvious as to be immediately acknowledged. This 

 is especially the case with my genus Bnllia compared with 

 Terebra : the shells of these two genera are so similar, that La- 

 marck 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 

 shght 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 BulUa 

 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 Terebrce have very small and 

 short tentacles, bearing the eyes near their tips. 



A second example of a similar kind is derived from the genus 

 Rostellarioy in which Lamarck includes the Strombus Pes Pell- 

 cani of Linnaeus. The animal of this shell has been figured by 

 Miiller, and very much resembles that of Bticcinum, having 

 long slender tentacles with the eyes sessile on the outer side of 

 their base ; while, as Dr Riippell informs me, the RosteUaria 

 curvirostris has an animal allied to Strombus, with the eyes on 

 very large peduncles, which give off from the middle of one of 



