Xii INTRODUCTION. [CH. 



has not been adopted by modern naturalists. Cuvier was 

 the first to restrict the term " Mollusca^^ to its present 

 meaning. Nearly all the MoUusca have a shelly cover- 

 ing, which protects either the whole of their bodies or 

 the more important organs. Even the Nudibranchs or 

 Sea-slugs, in their embryonic state, are provided with 

 a unispiral shell; and the Limacida or Land-slugs 

 possess a shelly plate, which is imbedded in the shield 

 or mantle. The term ^^ Conchology '' may be therefore 

 considered sufficiently appropriate to express the nature 

 of this science, especially when the original meaning of 

 the word is taken into account. I do not profess to be 

 conversant with the anatomy and morphology of the 

 MoUusca; and I must refer those who wish to studv 

 this part of the subject to the admirable treatises of 

 Cuvier, Milne-Edwards, Quatrefages, Troschel, Von Sie- 

 bold, Vrolik, and other Continental writers, as well as to 

 those of our own equally eminent countrymen, Owen, 

 Gray, Huxley, and Hancock. A complete knowledge of 

 the MoUusca is of course unattainable mthout such 

 study; but I must content myself with having made 

 such progress as time and opportunities have permitted. 



" Est quadam prodire tenus, si non datur ultra." 



Conformity of shells with their inhabitants. — The im- 

 portance of studying all the parts of the MoUusca is 

 undeniable, and especially for the pui^oose of arranging 

 them in generic and higher groups ; but the distinctive 

 characters afforded by the soft parts alone, which have 

 been erroneously termed by some writers the " animal," 

 cannot be much relied on in making out species. Phi- 

 hppi has completely demonstrated the insufficiency of 

 such characters in the case of the genus Rissoa ; and a 

 careful comparison of the descriptions given in the fol- 

 lowing pages of our Land and Freshwater MoUusca 



