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 eiforts 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 difference which exists between these two classes. 

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

 the shells, 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, in 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 limbs, 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 lamina. We have numberless instances 

 of shells, both sea and land, w^hich having been accidentally 

 broken, have been repaired by the animal: almost every 

 collection affords specimens of such mended shells. 



