92 ASHFORD : ON BANDED LAND SHELLS. 



change— one towards obliteration, the other towards diffusion. 

 In cases where the secretive power of the color glands has 

 become defective, less coloring matter has been deposited, the 

 beads have become more separated, first at the remote apex, then 

 gradually lower down the whorls, until in time individuals have 

 been produced totally devoid of vestiges of an upper band. On 

 the contrary in cases where the secretive organ has retained its 

 vigour, or for some reason or other has become increased in 

 energy the superabundant pigment supplied, unable to find access 

 to the enamelled ribs has diffused itself transversely into the 

 more porous structure of the intervening parts, producing those 

 blotches of_ color which give to so many shells their marbled 

 appearance. Examples of this occur in all the four species under 

 consideration, but are especially noticeable, as might be expected 

 from what has been already said, in a large percentage of 

 individuals of B. aciitus. Indeed instances are not wanting at 

 any rate in three of the species, in which this diffusion has 

 extended to the whole surface of the shell which then presents the 

 phenomenon of a monochrome divided only by the uncolored 

 striae, when present. I am not at all prepared to assert that the 

 irregular patches of color on all heliciform shells which possess it, 

 indicate zoned progenitors, it would require an infinitely better 

 acquaintance with exotic forms than I possess to make so general 

 an assertion. But I should not be suprised if facts are found to 

 support such a hypothesis. 



If these views be correct, as I think they are as regards the 

 species under consideration, it will follow that the variety bizo7ia 

 of B. acietiis, far from having been converted by circumstances 

 exterior or organic from the ordinary phase of that species to its 

 present pretty ornamentation, exhibits on the contrary that state 

 which retains most of the characteristics of far earlier forms. It 

 has been left behind in the march of change, outstripped by the 

 great mass of individuals of that species. Therefore in 



