16 THE SHELL. 



Anomia, the umbo frequently presents an exact imitation of the 

 surface to which the young shell originally adhered. 



Shells are composed of carbonate of lime, with a small pro- 

 portion of animal matter. The source of this lime is to be 

 looked for in their food. Modern inquiries into organic chemistry 

 have shown that vegetables derive their elements from the mineral 

 kingdom (air, water, and the soil), and animals theirs from the 

 vegetable. The sea-weed filters the salt water, and separates 

 lime as well as organic elements ; and lime is one of the most 

 abundant mineral matters in land plants. From this source the 

 mollusca obtain lime in abundance, and, indeed, we find frequent 

 instances of shells becoming unnaturally thickened through the 

 superabundance of this earth in their systems. On the other 

 hand, instances occur of thin and delicate-shelled varieties in 

 still, deep water, or on clay bottoms ; whilst in those districts 

 which are wholly destitute of lime, there are no mollusca. Helices 

 sometimes form cavities in limestone rocks, and M. Bouchard- 

 Chantereaux supposes the foot of the animal to exude an acid 

 solvent which effects the excavation ; but Dr. Fischer shows 

 that the jaw and tongue only are probably employed for this 

 purpose, and that the material excavated is used in the formation 

 of shell by the animal. So imperious is the necessity for this 

 material, that Limnseans brought up in captivity, devour the 

 shells of their companions in order to obtain it. M. Gassies, 

 who has raised many terrestrial mollusks, never fails to place 

 pieces of limestone in their cages, and these are eroded more or 

 less rapidly by the needs of the growing Helices. Omitting to 

 provide the limestone, the growth of the shell is retarded or 

 arrested. 



The texture of shells is various and characteristic. Some, 

 when broken, present a dull lustre like marble or china, and are 

 termed porcellanous ; others are pearly or nacreous ; some 

 have a fibrous structure ; some are horny, and others glassy and 

 translucent (i, 1). 



The nacreous shells are formed by alternate layers of very 

 thin membrane and carbonate of lime, but this alone does not 

 give the pearly lustre, which appears to depend on minute undu- 

 lations of the layers (i, 2). This lustre has been successfully 

 imitated on engraved steel buttons. Nacreous shells, when 

 polished, form " mother-of-pearl ; " when digested in weak acid 

 they leave a membranous residue which retains the original form 

 of the shell. This is the most easily destructible of shell- 

 textures, and in some geological formations we find only casts 

 of the nacreous shells, whilst those of fibrovis texture are com- 

 pletely preserved. 



Pearls are produced by many bivalves, especially by the 

 Oriental pearl -mussel Avicula niargaritifera (i, 9), and one of 



