SHELL-COLORATION IN BRITISH EXTRA-MARINE MOLLUSCA. G9 



ing results that very little can be deduced from the observations 

 made.* Perhaps it is the fact, not the nature, of the sudden 

 change which makes the difference. 



Generally speaking, one thing is obvious, that it is those 

 species which live most freely exposed to light which chiefly 

 develop pigment. This is in accordance with the general rule, 

 and, plainly, in the dark, sematic, cryptic, or epigamic colours, 

 would be of small service. 



Under certain circumstances a monochromatic shell can 

 adapt itself by cryptic, isochromatic coloration. Thus a case 

 of albinism in Pupa cylindracea on a white wall has been 

 recorded.! 



Simroth t suggests that the bands have something to do with 

 the large superficial blood-vessels, § and there is a good deal 

 of evidence which rather attracts one towards this view. 



Prof. Semper || gives a few very interesting cases of pseud- 

 aposematic coloration among land-snails from the Philippine 

 Islands, but this appears to be very rare, so far as our observa- 

 tions have at present gone. 



Finally, we may say that the whole matter is wrapped in 

 considerable obscurity, and, at any rate at present, it seems im- 

 possible to formulate any theory which will cover all cases. 

 Personally, I fancy that the colours of Tachea (and of Xerophila 

 partly) are cryptic, or are trying to be so. It is only by very 

 careful observations and accumulation of large masses of material 

 that such questions can be made out satisfactorily. To return to 

 the beginning again, I only hope that in these disconnected 

 jottings I have made it clear that Mr. J. W. Williams's idea may 

 be right, if a very ancient prototype is discussed (and his embryo- 

 logical reason would naturally only apply to this), while the 

 opposite view may turn out to be correct if more recent forms 

 are under consideration. 



Since writing the above, I have seen an interesting paper by 

 Mr. W. M. Webb, II in which he shows that Clausilia in Britain is 



* See ibid. i. pp. 91, foil. f J. W. Taylor, op. cit. i. p. 92. 



I See ibid. i. p. 96. 



§ Cf. the views of Alfred Taylor, ' Coloration in Animals.' 1886. 

 |] Op. cit. pp. 393, foil. 



11 " Protective Coloration in British Clausilias," in ' Science Gossip,' n. s. 

 vol. ii. No. 21, 227. 



