382 Zoological Society. 



Long. 1^. Lat. -^ poll. circ. 



Hab. in insula Camote. 



Legit H. Cuming in sylvis, foliis arborum adhserentem. 



In a great number of the beautiful land-shells of the Philippine 

 Islands, collected by Mr. Cuming, and herein-before described, the 

 pattern, upon immersion in water or other fluid, becomes entirely 

 obliterated till evaporation restores the colours to all their pristine 

 brilliancy. In the species now before us, the very reverse is the re- 

 sult of immersion. The external whitish porous epidermis which 

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

 immersed in water. Bui. velatus is described above, as it appears 

 on immersion, 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 moniliform appearance. 



I sent to Sir David Brewster, as the highest authority on such 

 subjects, four or five species of those land-shells from which the 

 pattern disappears upon immersion ; but I have not as yet forwarded 

 to him any upon which the colours come out when so treated. Sir 

 David has been so obliging as to send me the following letter, 

 which I now lay before the Society : — 



" Dear Sir, — I beg to return you my best thanks for. the very 

 interesting specimens of land-shells from the Philippine Islands, 

 which you have been so kind as to send me. The disappearance of 

 the white pattern by immersion in water or any other evaporable 

 fluid, and its subsequent reappearance when the shell is dry, are 

 phenomena perfectly analogous to those of hydrophanous opal, taba- 

 sheer, and other porous substances. 



" The phenomenon in the 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 pat- 

 tern 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 wherever 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 mem- 

 brane, 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 sepa- 

 rating surface of the water and the pores which contain it, so that 



