xii INTRODUCTION. [OH. 



has not been adopted by modern naturalists. Cuvier was 

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

 meaning. Nearly all the Mollusca 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 Limacidce 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 

 Mollusca; and I must refer those who wish to study 

 this part of the subject to the admirable treatises of 

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

 bald, 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 Mollusca is of course unattainable without such 

 study ; but I must content myself with having made 

 such progress as time and opportunities have permitted. 



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



Conformity of shells with their inhabitants. The im- 

 portance of studying all the parts of the Mollusca is 

 undeniable, and especially for the purpose 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- 

 lippi 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 Mollusca 



