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terials constituting each successive layer 

 are supplied by the organized fleshy sub- 

 stance called the mantle, and not by any 

 vessels belonging to the shell itself. The 

 connexion between the animal and the 

 shell may be regarded as mechanical 

 rather than vital ; for whatever portion of 

 vitality it might have possessed when first 

 deposited, all trace of that property soon 

 disappears. It is found that shells may 

 be impregnated with poisonous metallic 

 salts, and yet the animal suffer no in- 

 convenience. 



It is upon the exclusive shape of the 

 shell, and not the animal inhabiting it, 

 that the arrangement of conchology is 

 formed. Most shells are very different in 

 their young and adult state, both as 

 regards their form and colour. The Lin- 

 nsean arrangement of shells consists of 

 three orders, namely, multivalve, bivalve, 

 and univalve. 



Shells are found fossil in the most 

 ancient strata of the transition period 

 that contain any traces of organic life, 

 and many of these agree so closely with 

 the existing species, that we infer their 

 functions to have been the same, and that 

 they were inhabited by animals of form 

 and habits similar to those which fabri- 

 cate the living shells most nearly resem- 

 bling them. The most prolific source of 

 organic remains has been the accumula- 

 tion of the shelly coverings of animals 

 which occupied the bottom of the sea 

 during a long series of consecutive gene- 

 rations. A large proportion of the en- 

 tire substance of many strata is com- 

 posed of myriads of these shells, reduced 

 to a comminuted state by the long-con- 

 tinued movements of water. Minute 

 examination discloses occasionally pro- 

 digious accumulations of microscopic 

 shells, no less surprising by their abund- 

 ance, than their extreme minuteness ; the 

 mode in which they are sometimes crowd- 

 ed together, may be estimated from the 

 fact, that Soldani collected from less than 

 an ounce and a half of stone, found in the 

 hills of Tuscany, ten thousand four hun- 

 dred and fifty-four microscopic cham- 

 bered shells. Of several species of these 

 shells, four or five hundred weigh but a 

 single grain ; of one species, a thousand 

 individuals would scarcely weigh one 

 grain ; and great numbers of them could 

 pass through a paper in which holes have 

 been pricked with a needle of the smallest 

 size. 



The phenomenon of shells found in 

 rocks, at a great height above the sea, 

 has been attributed to several causes. By 

 some, it has been ascribed to a plastic 

 virtue in the soil ; by some, to fermen- 

 tation ; by some, to the influence of the 



celestial bodies ; by some, to the casual 

 passage of pilgrims with their scallops ; 

 by some, to birds feeding on shell-fish ; 

 but by all modern geologists, with one 

 consent, to the life and death of real mol- 

 lusca at the bottom of the sea, and a sub- 

 sequent alteration of the relative level of 

 the land and sea. 



The specific gravities of many shells have 

 been lately ascertained by Mr. De La 

 Beche. Those of land-shells ranged from 

 2-82 to 2-87 ; of fresh-water shells, from 

 279 to 2-82 ; those of marine shells, from 

 2-43 to 2-85. He adds, while the spe- 

 cific gravities of the land shells enu- 

 merated is generally greatest, the den- 

 sities of {he, floating marine shells is much 

 the smallest. The greatest observed den- 

 sity was that of a Jtelix, the smallest that 

 of an argonaut. The specific gravity of 

 all the land shells examined was greater 

 than that of Carrara marble ; in general 

 more approaching that of Arragonite. 

 The fresh- water and marine shells, with 

 the exception of the argonaut, nautilus, 

 ianthina, lithodomus, heliotis, and great 

 radiated crystalline teredo, exceeded Car- 

 rara marble in density. This marble and 

 the haliotis are of equal specific gravities. 

 The testacea, says Mr. Lyell, are by 

 far the most important class of organic 

 beings which have left their spoils in the 

 subaqueous deposites. There is scarcely 

 any great series of strata that does not 

 contain some marine or fresh-water 

 shells, and these fossils are often found 

 so entire, especially in the tertiary forma- 

 tions, that when disengaged from the 

 gangue, or matrix, they have all the ap- 

 pearance of having been just procured 

 from the sea. Their colour, indeed, is 

 usually gone, but the parts whereon spe- 

 cific characters are founded, remain un- 

 impaired ; and though the animals them- 

 selves have disappeared, their form and 

 habits can generally be inferred from the 

 shell which covered them. 



From the proportions which fossil shells 

 bear to marine shells of existing species, 

 M. Deshayes and Mr. Lyell have proposed 

 a fourfold division of the marine forma- 

 tions of the tertiary series. The total 

 number of known fossil shells, at the time 

 Mr. Lyell wrote his fourth edition of 

 Principles of Geology, in the tertiary 

 series was 3,036. Of these 1,238 are 

 found in the eocene; 1,021 in the mio- 

 cene ; and 777 in the older, and newer 

 pliocene divisions. The numerical pro- 

 portions of recent to extinct species may 

 be thus expressed. In the newer pliocene 

 period 90 to 95 per cent, are of recent 

 species ; in the older pliocene, 35 to 50 ; 

 in the miocene period 1 8 ; in the eocene 

 period 3 per cent, only are of recent 



