12 INTRODUCTORY. 



dread of the mariner, appears an insignificant instrument in the 

 hand of Providence, in humbling the glory and pride of man, in 

 demolishing, by its unseen labours, the noblest efforts of his in- 

 genuity and skill. 



Conchology has been sometimes confounded with Crustace- 

 ology, but the slightest attention to the subject will at once dis- 

 cover the obvious difference which exists between these two 

 classes. Nature has not only well defined them by the compo- 

 sition of the shells, but also by a manifest difference in the con- 

 struction of the animals which inhabit them. Testaceous shells 

 are composed of carbonate of lime, combined with a small por- 

 tion of gelatinous matter ; while those of the Crustacea are com- 

 posed of phosphate of lime, along with the animal matter. Tes- 

 taceous shells are, in general, permanent coverings for the in- 

 habitants, and the animal is of a soft and simple nature, without 

 bones of any kind ; and attached to its domicile by a certain ad- 

 hesive principle, possessed by some of the muscles. On the 

 other hand, shells of crustaceous animals are cast, and renewed 

 aimually. The animals are of a fibrous texture, with articulated 

 limbs, and covered, as it were, in a coat of mail. Besides, the 

 shells of crustaceous animals are produced all at one ; those of 

 the Testacea, evidently are formed by the animal gradually adding 

 to them either annually or at least periodically ; as may be dis- 

 tinctly seen in the common muscle : and all shells, strictly 

 so called, are composed of layers, which is not the case with 

 the coverings of Crabs and their congeners. This may be easily 

 proved by taking a shell and filing it slowly, when the different 

 layers are very perceptible ; or put a Muscle- shell into the fire, 

 when it will shortly begin to crackle and divide into separate 

 laminae. We have numberless instances of shells, both sea and 

 land, which have been broken, and firmly mended by the 

 animal ; almost every collection affords specimens of mended 

 shells. 



The terms Conchology and Testaceology include all animals 

 which have a testaceous covering, whether they inhabit the sea, 

 land, or fresh-water. 



Chemists have divided shells into two classes. The first are 

 usually of a compact texture, resembling porcelain, with an en- 

 amelled surface, and are in general beautifully variegated. Those 

 composing this class are called, porcclaneous shells, which in- 



