338 MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 



many cases, since they can scarcely be seen while the shell is 

 living and until after the epidermis has been removed. There 

 is another way in which patterns are produced in some land 

 shells, viz., where the normally coloured opaque bands 

 become translucent, and allow the dark colour of the animal 

 to show through. These translucent bands show a difference 

 in the structure of these portions of the shell, and are not 

 merely thinner because the pigment cells are absent, as in these 

 cases there are plenty of specimens in which there is the same 

 absence of the pigment cells while the shell itself is not banded 

 in any way. No species shows this better than the common 

 Helix ericetoruin. 



Nature is too fertile in resource to invariably produce the 

 same effects in one way, for the colour and patterns of some 

 species are due to the epidermis, the epidermis being formed in 

 a manner similar to the shelly portion of the mantle. Some 

 Bulhni have peculiar epidermis. ' It is a common feature in the 

 Phillipine species,' says Mr. Lovell Reeve, ' that the varieties of 

 pattern which constitute their chief ornament reside only in the 

 epidermis. The colours of the shell rarely describe any kind 

 of configuration. They are mostly blended into a uniform tint, 

 over which a fanciful pattern is produced by the epidermis 

 forming a double porous membrane in some places and a single 

 one only in others, developed moreover with the same con- 

 tinuous regularity as the textile markings of a Vohite or Cowry. 

 This phenomenon is easily detected by immersing the shell in 

 water, when the light portion or upper porous layer of the 

 epidermis becomes saturated, and the ground colour of the shell 

 is seen through it ; as the moisture evaporates the epidermis 

 resumes its light appearance. In some cases the epidermis 

 alone gives colour to the shell, and when this is removed the 

 shell is left of a uniform white of greater or less purity.' 



While on the subject of colour, it will be convenient to add 

 a few remarks on the subject of colour variation and the way in 

 which it is produced. Speaking generally, shells are the more 



J.C. , vii. , Jan. 1894. 



