TAPES. 63 



This species shows how difficult it is to found arrange- 

 ments on any one peculiarity, for no distinction would seem 

 to be more important between two species or genera, or even 

 families, than that which would result from contrary habits 

 of the animals ; and yet such contrary habits exist, and are 

 shown by contrary characters of the shells, even in different 

 individuals of the same species. The following interesting 

 observations on the instance before us are by the authors 

 of the ' History of British MoUusca.^ 



'^We are compelled by the force of facts to come to the 

 conclusion that the two are identical ; and though the shells 

 of each seem to present greater differences than their ani- 

 mals, still the principal form and outline of both, with their 

 strise of growth and decussations, are essentially of the same 

 character ; the variations are mere modifications of similar 

 elements dependent on habitat, deprivation of light, or ex- 

 posure to it ; for when the shells are imbedded in the cavities 

 of rocks they are subject to variations of form, and being 

 deprived of light, they are usually colourless and without 

 markings ; and not being rubbed by the action of the waves 

 amongst the shingly beaches, their striae are sharper and 

 more foHaceous. Amongst shingle, where they are free, or, 

 if the current be strong, attached by their filaments to stones. 



