36 



every period of activity, and remain like a grove of spines 

 on the back of the shell. In the Terebra and other shells 

 where the lip is thin there is nothing to mark the succes- 

 sive periods of growth, and in others, though the lip may 

 be thickened and niarginated, it does not appear to be thus 

 developed until the shell is complete ; in some species of 

 Cassis, for example, though the lip is greatly thickened, 

 the back is smooth. In the Pteroceras there is no appear- 

 ance of the remarkable horns which garnish the aperture 

 until the shell has attained its full growth. In many cases 

 after the atnimal and the shell have attained th eir full de- 

 velopment the action of the mantle still continues to add 

 fresh materials, and old shells are often extensively solid 

 and massive. 



Nothing has yet been said of the division of the classes 

 into orders and sections, nor will it be necessary to say 

 more than as the forms of the shells may be affected. 

 Naturalists group the molluscs in accordance with the struc- 

 ture of the animals, making very little account of the shells. 

 Hence it arises that even in one and the same genus the shells 

 often exhibit the greatest dissimilarity in form and magni- 

 tude, as may be observed by inspecting the two groufis on 

 the table of the Murex, the typical genus of the family of 

 the Muricidae, and the Volute, the typical genus of the 

 Volutid». Nevertheless there are family likenessess, as will 

 be noted presently. ( in the other hand, there are shells 

 closely resembling one another, though the animals are en- 

 tirely distinct. The Siphonaria, for example, which is a 

 land shell, is so like a Limpet, that it requires a close inspec- 

 tion of the interior markings to distinguish them. The 

 animal structure is referable principally to the breath- 

 ing organs : these divide the molluscs into two great 

 groups— the Branchiata, those which breathe water 

 by means of branchiae or gills, like fishes, and the 

 Pulraoniftra, those which breathe air by means of lungs, 

 like Christians, and the rest of the land animals. Some 

 ^ arieties in the structure and position of the gills divide the 



