62 THE ZOOLOGIST. 



ON SHELL-COLORATION IN BRITISH EXTRA-MARINE 



MOLLUSCA. 



By Arthur E. Boycott. 



A good deal of somewhat acrimonious discussion went on 

 some years ago in one of our scientific monthlies* with regard 

 to the original colouring of British Land and Freshwater 

 Mollusca. There were represented there, and elsewhere, two 

 views: first, that "Helix cantiana, cartusiana, &c, were once 

 banded species ;"f this view was held notably by J. W. Taylor 

 and C. Ashford ; secondly, to J. W. Williams there seemed 

 " scarcely any foundation for such a supposition." The chief 

 reasons which induced the latter authority to hold the views he 

 expressed seem to have been (1) that the primitive embryo shell 

 is colourless : (2) that the nucleus in banded species is usually 

 colourless ; (3) that freshwater forms are usually unicolorous, and 

 these have been subjected to less change of environment.! 



In the following remarks I may be able to suggest that both 

 views are, in a way, correct ; the difference perhaps lies only in 

 what is understood by the term " once." The meaning of such a 

 phrase as "original ancestors" may, and possibly has, led to much 

 confusion. With regard to the prototype of the large group of 

 Mollusca, or indeed of the Gastropoda only, the argument would 

 be somewhat complicated. On this question evidence could only 

 exist in a very hazy form, and any conclusion arrived at would 

 be but dubious. But, judging from the general aspects of the 

 question, it would seem probable that the first molluscan shell 

 was horn-coloured, colourless, or white. For it would appear 

 obvious that the mollusc would first get the trick of secreting a 

 shell, and afterwards the trick of elaborating it for some useful 

 purpose. Whether it were white, or transparent and colourless, 

 would probably depend chiefly on the molecular condition of the 



* ' Science Gossip,' vol. xxvi. p. 178 (J. W. Williams) ; ibid. p. 233 

 (S. Pace) ; ibid. p. 241 (S. C. Fryer) ; ibid. p. 274 (J. W. Williams) ; vol. xxvii. 

 p. 73 (J. W. Williams) ; ibid. p. 121 (W. M. Webb). A bibliography of the 

 subject is given by J. W. Williams in vol. xxvi. p. 44. 



f " Valedictory Address," in ' Journal of Conch.,' April, 1888. 



+ ' Land and Freshwater Shells,' 1889, p. 19. 



