116 



CONCHOLOGY. 



This epidermis is formed of a dried mucus, or horny 

 matter, forming 1 a covering more or less thick in 

 some shells it is more or less united, in others thick 

 like fur, or disposed in ridges. In bivalves this part 

 is formed of the same substance as the ligament, and 

 sometimes totally envelopes the valves, as in the Mt/a, 

 Solen, &c. Its growth keeps pace with the increase 

 of the shell, whether it afterwards remains upon it or 

 not. 



The opercula of shells, though not absolutely a 

 portion of them, nevertheless belong to the animal 

 possessing them, and which, in the modern arrange- 

 ment of Malacology, must be studied with their cover- 

 ings ; these interesting parts have been scientifically 

 treated by Mr. Gray, F.R.S. ; they tend greatly to 

 assist in the classification of molluscs, to point out 

 their structure, peculiar habits, and to form an admir- 

 able guide to localities, otherwise without positive 

 indications. 



The colouring matter of shells may be said not 

 properly to belong to them : it is always extremely 

 superficial, and is produced by the coloured pigment 

 secreted on the surface of some portion of the animal's 

 body ; these are coloured atoms, deposited above the 

 calcareous matter, and are of a different nature, since 

 they form no necessary portion of the shell's strength, 

 disappearing under circumstances of age, injury by 

 artificial heat, the application of acids or other causes. 

 The colours are always more vivid in young animals, 

 and the recent portions produced by them. Anatomists 

 have proved, that it is only the limb or anterior edge 

 of the animal's mantle which produces these colouring 

 molecules, for, if a hole is made in any other part of 

 the shell than at its edge, the reparation is made with 

 matter quite colourless, and forms a white spot. In 

 the snail (Helir ncmoralis'), which is so elegantly zoned 

 with bands of black upon a yellow ground, it may 

 easily be seen that the portion of the animal's body 

 corresponding with these, presents a tinge of that 

 colour, so that a fracture made there, on the extreme 

 edge of the shell, is restored by a similarly coloured 

 portion to that injured or removed, provided the 

 broken part is altogether removed, otherwise, we see 

 in the snail, inhumanly crushed to a shapeless mass, 

 repeated proofs of the reparations made by the animal 

 being perfectly colourless, every where but at the lip 

 of the shell ; in this state examples may occasionally 

 be met with, so industriously mended after what was 

 deemed a total destruction of the animal, that its shell 

 appeared made up of numberless patches, as variously 

 disposed as Harlequin's jacket. 



Though we have no certain proof that all other 

 shells, whose coloured zones run spirally from the 

 summits, are similarly formed, analogous reasoning 

 authorises us to draw that conclusion ; but with 

 species whose colouring matter is externally deposited 

 in oval, square or irregularly formed spots, and parti- 

 cularly where the transverse bands, or stripes, are in 

 the direction of the additions to the shell's growth, 

 that is, perpendicular from the summit to the base, 

 the analogy is not so evident-; without we subscribe 

 to the doctrine of some naturalists, that there is a 

 change, a displacement, either regular or not, in the 

 colouring parts of the animal's mantle, under peculiar 

 circumstances of structure, constant and undeviating 

 as any other of nature's laws. These phenomena 

 admit, however, of much speculation, arid can only 

 be explained by a long series of accurate observa- 

 tions of the animals themselves in the different 



stages of their growth, a thing so difficult to come at, 

 and requiring a particular train of study to benefit 

 from, when within our reach, that we must content 

 ourselves with well-grounded supposition, in the 

 absence of positive proof. 



We have just, said that the coloration of shells is 

 constantly superficial ; this, however, must be qualified 

 when speaking of such shells as we have described 

 under the name of porcelanic, the cowry, and some 

 species of olive. The formation of the cowry hav- 

 ing been explained, it is obvious that, during the 

 animal's young state, the colouring matter is external, 

 and proceeds from the same causes as in other 

 shells. This coloration is due to the edges of the 

 mantle, and is gradually made as the animal increases 

 in size ; but, probably at a later period, when it has 

 attained its complete maturity, the cutaneous append- 

 ages on each side of the animal's body, when it climbs, 

 are elevated on the bick of the shell, depositing that 

 vitreous or glassy ivory coating, which gradually 

 thickens it, and at the same time leaves a colouring 

 matter, constantly presenting a different disposition to 

 the first. It must then be admitted, that the upper 

 part of these cutaneous lobes possess intervals where 

 the pigment is coloured, imbuing the cretaceous 

 matter exhaled from them ; and, as it is very unlikely, 

 indeed almost impossible, that these spaces or inter- 

 vals in the development of the lobes, should exactly 

 cover the part on which the colour is deposited a 

 second time, it is easily to be conceived that the 

 markings are necessarily irregular, and can never 

 exhibit either bands or any other running design. An 

 examination of the leopard cowry, in its adult state, 

 will render this subject easily understood, and in the 

 map cowry, though a more rare shell to meet with, it 

 is yet more strikingly illustrated ; in this, the space not 

 touched, or totally covered, by the cutaneous lobes of 

 the animal's body, is distinctly marked by well-defined 

 lines, distributed in various curves like the face of a 

 map, from whence its appropriate name. This is 

 visible in the common and other cowries, but not so 

 strongly marked. 



We must also remark, that the colouring matter of 

 some shells is occasionally on the internal surface, of 

 bivalves in particular. In these exceptions to the 

 general rule, the effect may be attributed, we think, 

 to the matter deposited, and it appears to be produced 

 by an impregnation of colour, gradually extending in 

 depth as well as surface, occasioned probably by some 

 humour of the animal's body, produced in any organ, 

 the immediate contact 6f which with the shell, tinges 

 it of the colour of that secreted fluid. This appears to 

 apply to the yellow and brown interior of some univalve 

 shells, which certainly is caused by the liver of the 

 animal being in contact with them ; the JantMna is 

 also coloured by similar causes. It may, however, 

 be generally asserted, that it is extremely rare to find 

 the component strata of shells otherwise than white ; 

 they are, however, sometimes, as has been stated, 

 both externally and internally, tinged with colouring 

 matter. 



We said that light and heat exercise a prodigious 

 influence in the colouration of shells, as well as it 

 does in the other portions of creation. All such 

 as are inclosed in the interior of the animal, in holes 

 of wood, chalk, or in other situations from whence 

 they never emerge, are constantly white; but another 

 more striking example may be adduced from certain 

 bivalve shells, which being fixed more or less h^rf- 



