TERRESTRIAL SHELLS. 487 



great, that they were not unworthy to play their part with the 

 crocodiles, the Ichthyosauri and Plesiosauri of a former world. 



I draw my letters on Terminology to a conclusion with 

 some explanations that apply to shells in general, and which 

 have not been previously adverted to. Linnaeus classified 

 the Testacea into the terrestrial, the fluviatile, the lacus- 

 trine, the littoral, the maritime, and the pelagic ; * and the 

 distinctions are so important that they should be indicated in 

 every good description of a species. 



Terrestrial or land-shells, are all univalve and turbinate ; 

 and, with rare exceptions, they are non-operculate. So far 

 as I can remember, none of them have the nacred or pearly 

 structure ; and none of them have true varices or spinous 

 ribs, or indeed any other external sculpture than spiral 

 striae or transverse lines of growth. The texture of the shell 

 is light, and its beautiful colours are mostly disposed in well- 

 defined bands ; so that a conchologist soon learns to distin- 

 guish a land shell from its peculiar texture and colouring, 

 and general aspect. Some Bulimi have a peculiar epidermis. 

 " It is a curious feature in the Philippine species," says Mr. 

 Lovell Reeve, " that the varieties of pattern which consti- 

 tute their chief ornament reside only in the epidermis. The 

 colours of the shell rarely describe any sort 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 continu- 

 ous regularity as the textile marking of a Volute or Cone. 

 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. Sir David Brewster, 

 in reply to a letter from Mr. Broderip on this subject, says : 

 ' 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 membrane ; in the same manner as hydro- 

 phanous opal has become white, from the expulsion' of its 

 water of crystallization." j 



* Syst. Nat. 1070. 



t Ann. and Ma». N. Hist. ser. 2. i. 271. 



