S§ALAEIx\. 165 



S. communis has the ridges nearly uniform in size, and those 

 of one whorl lock into and are continuous with those 

 of the other; the colouring is pale fawn or nearly 

 white between the varices, variegated with purple 

 touches ; the varices are white, with purple spots in 

 bands. 



S. Tiirtoiiis has the whorls rounder, the varices not strictly 

 continuous, thin in the upper whorls, irregularly thick- 

 ened and flattened in the lower ; general aspect more 

 brown than in 8. coynmunis. 



S. clathratula : small, white, with sharp varices. 



S. Trevelyana: fawn-coloured, not variegated with thin, 

 uniform varices. 



S. Grcenlandicaj rarely found on our coasts, has rather flat- 

 tened ribs, with sjnral grooves bekveen them. 



