14 INTRODUCTION. 



the field, and marks its progress by the destruction of some 

 of the fairest forms and most useful products of the vege- 

 table kingdom; and the Ship-worm, the dread of the mariner, 

 appears an insignificant instrument in the hand of Provi- 

 dence, in humbling the glory and pride of man, in demolish- 

 ing, by its unseen labours, the noblest efforts of his ingenuity 

 and skill. 



Conchology, or the study of shells, has been sometimes 

 confounded with Crustaceology, or that of Crabs ; but the 

 slightest attention to the subject will at once discover the 

 obvious dJiTerence which exists between these two classes. 

 Nature has not only well-defined them by the composition of 

 the shelU, but also by a manifest difference in the construc- 

 tion of the animals which inhabit them. Shells, properly so 

 called, are composed of carbonate of lime, combined with a 

 small portion of gelatinous matter ; while those of the Crus- 

 tacea are composed of phosphate of lime, along with the 

 animal matter. Shells are, iji general, permanent coverings 

 for their inhabitants ; and the animal is of a soft and simple 

 nature, without bones of any kind, and attached to its domi- 

 cile by a certain adhesive property or power, possessed by 

 some of the muscles. On the other hand, shells of crustaceous 

 animals are cast, and renewed annually. These animals are 

 of a fibrous texture, with articulated Ijmbs, and covered, as 

 it were, in a coat of mail. Besides, the shells of crustace- 

 ous animals are produced all at once ; those of the Testacea, 

 are evidently formed by the animal gradually adding to them 

 either annually or at least periodically; as may be distinctly 

 seen in the common muscle: and all shells, strictly so called, 

 are composed of layers, which is not the case with the cover- 

 ings 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 having been accidentally 

 broken, have been repaired by the animal: almost every 

 collection affords specimens of such mended shells- 



