THE COLOUR OF SHELLS. 425 



case with Oliva utriculus, which is often sokl in that state as 

 a different shelL There is reason to believe that Lamarck 

 was deceived hy a specimen which had been so mutilated, 

 and which he described as a distinct species, under the name 

 of O. harpiformis. 



The belts across the whorls of the Olivse and Ancillariae 

 have the coloured matter deposited on their inner surface. 

 In a few shells, the Oliva porphyria for example, some of 

 the more distinct colovu-ed lines even form raised ridges on 

 the outer surface ; and in some others the inner layer is 

 darker than the outer. Thus in Oliva tessellata and O. line- 

 olata, and some Cowries, the inner layer is purple ; in O. 

 spidula, brown ; in Turbo chrysostomus and T. nicobaricus, 

 bright golden; in Capulus hungaricus, in Strombus gigas, 

 and in many others, pink : but it is generally much paler, 

 and in the greater number of shells white or colourless. 



Some brown shells, such as the Volutes, become white 

 when touched with a hot iron : there was formerly in the 

 Museum at Paris a specimen of a Melon, marked in this 

 manner with close series of white spots. The purple colour 

 of some shells is also changed under a similar treatment to 

 dusky red ; and it is by this process that the red spots are 

 formed on the polished muscle-shells and uncoated Nutmeg 

 Cowries, which are, or were, so abundant in the shops. A 

 very curious effect is produced by ink on some purple shells, 

 which I have seen only in individuals of that colour. On 

 the receipt of the Cracherode collection of shells at the Bri- 

 tish Musemn, my uncle. Dr. Gray, wrote on each of them 

 with a pen and ink the number of the catalogue and the 

 name of the species. In some instances, although the ink 

 has been washed off, the name and number are still distinctly 

 visible, forming an evenly raised letter as broad as the ink 

 line, and slightly interrupted, as if by bubbles, in a few 

 places. This is particularly visible in the specimen of Solen 

 diphos. No. 186, in that collection. The effect, however, is 

 not produced in all shells of this colour, nor even in some of 

 the same natural genus which were written upon at the same 

 time ; but as I have observed it in a few other purple shells, 

 and as I have not met with it in any of a different colour, I 

 am inclined to think that the nature of the colouring matter 

 may have some share in its production. 



3. The Structure of Shells. 



Shells exhibit, when examined, two very distinct kinds of 

 structure : in the one case the calcareous particles of which 



