one fleshy envelope, not retractile within the shell, but 

 always in part exposed. Yenerupis approaches nearer to 

 Venus in the number of its teeth, which, however, are not 

 divaricated as in the latter genus. 



The means by which these animals penetrate dense calca- 

 reous substances, has been discussed by many able writers, 

 without a satisfactory solution of the problem. Some 

 have supposed that the operation is effected by the friction 

 of the valves of the shell ; but the valves of some species 

 are very thin and not so dense as the substance they pene- 

 trate, and never exhibit any abrasion of their attenuated 

 edges. Others contend that a peculiar acid or solvent must 

 be secreted by some appropriate organ, which dissolves 

 the rock by a chemical action ; but neither anatomy nor 

 chemistry have exhibited proofs in support of this opinion, 

 and in this state of uncertainty we are stiil left to conjec- 

 ture and analogy. We know that the power of penetra- 

 ting calcareous substances, as well as wood and extremely 

 dense earth, is not confined to animals of this family, but 

 that many others bore through shells to devour the inha- 

 bitant, with too small a hole to admit any part of their own 

 shell, and numerous other species as their whirls revolve 

 in the growth, remove the asperities of the preceding vo- 

 lution as the aperture approaches them. This effect is ob- 

 servable in almost all rough univalve shells ; some indeed 

 cover their slight inequalities with the calcareous deposite 

 of the labium, but whenever the inequality is prominent, 

 it is sure to be removed at the aperture, and it would seem 

 that the operation may possibly be, in some instances at 

 least, effected by the constant action of the soft parts of 

 the animal, or by the agency of absorbents acting on the 



PI, 60. 



