96 MICROSCOPICAL MEMORANDA. 



great number of the beautiful land-shells of the Philippine Islands, col- 

 lected by Mr. Cuming, the pattern, upon immersion in water or other 

 fluid, becomes entirely obliterated till evaporation restores the colours 

 to all their pristine brilliancy. In this species, the very reverse is the 

 result of the immersion. The external whitish porous epidermis which 

 veils the shell when dry, suffers the bright colours to shine out when 

 immersed in water. B. velatus is described above, as it appears on im- 

 mersion, and before it becomes dry ; but in the latter state the beauties 

 of the shell are shrouded, and the colour of the sutural bands, peeping 

 out between interstices in the epidermis, gives to these bands a monili- 

 form appearance. Mr. Broderip forwarded to Sir D. Brewster, four or 

 five species of those land-shells, from which the pattern disappears upon 

 immersion ; but he has not as yet forwarded to him any upon which 

 the colours come out when so treated. The following is an extract 

 from Sir D. Brewster's letter on the subject : " The disappearance of 

 the white pattern by immersion in water, or any other evaporable fluid, 

 and its subsequent re-appearance when the shell is dry, are phenomena 

 perfectly analogous to those of hydrophanous opal, tabasheer, and other 

 porous substances. The phenomenon in land- shells is still more 

 beautiful when we examine them by transmitted light. The pattern, 

 which is white by reflected light, is dark by transmitted light, and vice 

 versa. This is particularly beautiful in the Helix pulcherrima, where 

 the ground of the white pattern is almost black by reflected light, and 

 of a light reddish colour by transmitted light, the pattern which is white 

 by reflection having a dark red colour by transmitted light. In all these 

 shells the difference of structure by which the pattern is produced, does 

 not exist in the shell, but in the epidermis, and hence the pattern may 

 be wholly obliterated by removing the epidermis. It appears to me, 

 from very careful observations, that the epidermis consists of two layers, 

 and that it is only the upper layer which is porous whenever the pattern 

 is white. These white or porous portions of the epidermis differ from 

 the other parts of the upper layer only in having been deprived of, or 

 in never having possessed, the element which gives transparency to the 

 membrane, in the same manner as hydrophanous opal has become white, 

 from the expulsion of its water of crystallization. When the shell is 

 immersed in water or any other fluid, the fluid enters the pores of the 

 white epidermis, and having nearly the same refractive power as the 

 epidermis, no light is reflected at the separating surface of the water and 

 the pores which contain it, so that the light passes through the mem- 

 brane, which thus loses its white appearance. When the water escapes 

 from the pores by evaporation, or is driven from them by heat, the 

 membrane again reflects white light from the numerous surfaces of its 

 pores. 



" As the colouring matter resides in the shell itself, its peculiar co- 

 lour is seen through the epidermis as distinctly where it is porous as 

 where it is not porous, when the porous portion has been rendered 

 transparent by the absorption of a fluid. 



"If we apply oil or varnish to the white pattern, we may obliterate 

 it permanently, or we may change it into a pattern entirely different 

 from the original one." Proc. ZooL Soc. Land., Feb. 9th, 1841. 



