go ASHFORD : ON BANDED LAND SHELLS. 



and the other below the periphery, the latter being multiple, and 

 (2) a surface much less sculptured, or diversified by stride, ridges, 

 plaits or wrinkles than even the smoothest of the four at the 

 present day. 



Circumstances, the causes of which are not evident, have in 

 the course of time greatly modified the outer surface of the shell. 

 This I shall endeavour to establish. 



It will be admitted by all who have even casually noticed a 

 few score of these shells, especially if collected from different 

 localities, that a change in the colored markings has taken place. 

 When we see a series of forms differing much in the extremes, but 

 having the characteristics of individuals gliding imperceptibly into 

 those of their neighbors, the idea first suggested to the mind is 

 that of progressive change. Now the tendency of the modifica- 

 tions of color has been towards a gradual breaking-up of the bands 

 into beads, blotches or multiple zones. Its cause, at least as far 

 as the beads and blotches are concerned, must be sought for I 

 think in the more or less coarse ridges in the line of growth. An 

 examination with the lens will show that those parts of the shell 

 which are interposed between the segments of the broken zone 

 are hard and often enamelled processes rising above the level of 

 the colored parts ; and such an examination can hardly fail to 

 bring the conviction that it is the growth of these processes which 

 has been the cause of the interception referred to. The existence 

 of stride then and the fragmentary state of the zones stand to one 

 another in the relation of cause and effect. But we admit that 

 the condition of the zones has undergone a change, therefore the 

 state of the striae has changed also. 



This being the case, it seems fair to assume that the first 

 change which took place in the distinctly zoned progenitors 

 referred to, manifested itself in the gradual development or 

 increased development of external stride, of which ridges such as 

 diversify B. acutus and other shells are only a more pronounced 



