CONCHOLOGY. 



115 



being thus completely covered with the second layer 

 the original colours are concealed, and if the same 

 shell were examined at the different periods of its 

 growth or increase, it would appear like two distinct 

 species. In the genus Cyprcea (common cowry), this 

 is strikingly visible ; the young shell is thin ; the 

 spire formed by its convolutions very distinctly 

 marked, and the toothed, or grooved aperture, not at 

 all indicated : in this stage it closely resembles some 

 species of cones, and by early writers on concliology 

 has been so classed. As the external deposition of 

 matter continues, the thickness of the shell increases, 

 the spire becomes less and less, till it is altogether 

 obliterated, and an indentation is formed on the part it 

 occupied, by the repeated coverings it has received ; 

 the matter also undergoes a change of quality ; 

 becomes more glassy or transparent, for it is to be 

 remarked, that when a shell has attained the largest 

 growth it is susceptible of, the skin of the animal 

 appears to produce a greater quantity of calcareous, 

 and less of the gelatinous matter ; the atoms com- 

 posing it are no longer in regular layers or leafy, they 

 have become extremely compressed and neaped 

 together, their structure vitreous, and assuming a 



i? Shell. 



Adult Shell. 



RJBA EXANTHEMA, 



brilliant polish, from the continual friction of the parts 

 of the mantle, as they pass constantly over the surface 

 of the shell during the life of the animal ; this is 

 observable in all univalve shells, most particularly so 

 in the cowry and its congeners ; each addition to the 

 rolled form of the shell is rendered thicker, and, when 

 it has reached its full period of maturity, the lips of 

 the opening are become extremely thick, and with 

 those grooves which admit certain portions of the 

 animal's body to pass through. Thus, the different 

 modes of increase may readily be understood, and it 

 is plainly quite distinct, from that which we have 

 described as taking place in the snail and similarly 

 constructed shells, in the one, a shell may familiarly be 

 termed built by small additions of matter to its first 

 formation, placed as it were in lines, edge to edge, as 

 any thin substance would be rendered wider by care- 

 fully laying a coat of varnish on its edge, adding various 

 others in succession, as each previous one becomes 

 hardened ; while in the porcelanous shells, the depo- 

 sition of matter is layer upon layer, over the whole 



surface of its broadest expanse, or in more scientific 

 terms, stratum super-stratum. 



The lamellar structure of shells may easily be dis- 

 covered by exposing the common oysters, and such as 

 are so constructed, to the action of heat, when the 

 animal matter which cemented the calcareous layers 

 is destroyed, and they are shown in a foliaceous form. 

 The fracture of the shell is another fact illustrative ot' 

 its formation ; an oyster never breaks short off, but in 

 thin scales or portions, while the snail and bivalve 

 shells break short off in the direction of their additions 

 to the edge of the shell, some more readily than 

 others, according to the density of the matter com- 

 posing them. In the matchless Museum of the Royal 

 College of Surgeons, there are some beautiful prepar- 

 ations of shells and madrepores, in which the calcare- 

 ous portion of matter has been absorbed by a chemical 

 process, not affecting the animal matter, which remains 

 exhibiting the precise form of the shell, by far the 

 greater proportion of which it constituted. The 

 chemical analysis of shells has been carefully made, 

 and its general result proves them to be formed of a 

 gelatinous matter hardened by a calcareous salt. The 

 species which contain the largest portion of animal 

 matter, appear to be those of a fibrous and pearly 

 structure, in these exist sub-carbonate of lime and 

 coagulated albumine. Mother-of-pearl itself is com- 

 posed of sixty-six parts of the first and thirty-four of 

 the latter. The shells of oysters contain much less 

 animal matter, and that which does exist, more 

 resembles a gelatinous substance ; Vauqueline dis- 

 covered in it sub-carbonate of lime and phosphate of 

 of lime, with sub-carbonate of magnesia and oxide of 

 iron. 



The Patella (limpet), which is formed of an ex- 

 tremely compact lamellar structure, approximates the 

 most, in its chemical composition, those shells, which 

 are, in general, vitreous, or, as it is termed Porcellanic ; 

 these contain only a small proportion of animal matter, 

 but a very large one of sub-carbonate of lime, without 

 any traces of phosphate or sulphate of lime. Human 

 chemistry has brought to light many wonderful pheno- 

 mena, and has gone some length in elucidating them ; 

 but great as these discoveries have been, they are no 

 more than as distant planets in the wide-spread firma- 

 ment, when compared to the operations incessantly 

 performing in nature's laboratory. By the aid of 

 chemistry we may, indeed, decompose nearly every 

 substance in nature dissolve some of them into air 

 collect the gases again, and prove, by the test of the 

 most accurate weights, that no portion of the original 

 bulk is lost, though rendered invisible to the eye ; it 

 is therefore, almost demonstrable, that the globe 

 weighs no more at this moment than it did at its first 

 creation, the destruction of some portions of it con- 

 tributing to the formation of others, nothing being 

 lost. But here, human science stops ; we cannot take 

 the elementary atoms and fashion them into a rude 

 oyster or rough pebble ; in this the master touch is 

 wanted, all the accumulated wisdom of ages past, or 

 of generations to come, can do no more, can go no 

 farther, than to approach nearer and nearer, by slow 

 degrees, the impenetrable barrier eternally fixed be- 

 tween omnipotent wisdom and finite judgment. 



Another constituent portion of many shells is an 

 external covering called the epidermis, the precise 

 use of which does not appear satisfactorily explained ; 

 it is evidently the epidermis of the animal's skin 

 beneath which the shelly matter has been deposited 



