RISSOA. 115 



tint by any peculiar colouring. The pillar-lip, whose 

 attached or inner border is a little raised, is straightish, 

 and almost perpendicular; it is remarkably narrow, occu- 

 pies a considerable portion of the inner lip, and is more 

 usually flanked by a kind of subunibilical crevice. The 

 usual sizes range from the seventh of an inch and half 

 as much in breadth, to the tenth of an inch and two- 

 thirds as much in width. Some narrow transparent 

 smoothish streaked examples from Rothsay, however, 

 measured the fifth of an inch long. 



In the shape of the mouth and pillar it resembles the R. 

 albella of Loven ; yet as all the examples of it with which 

 we have been favoured by the author present a more 

 abbreviated figure, stronger ribs, and a shorter spire, we 

 have not cited that species as identical. Nevertheless, we 

 are far from sure that this is not the case. 



We are disposed to group together our specimens into 

 four principal varieties, each of which runs so closely into 

 the succeeding one, that it is difficult for those who pos- 

 sess a large number of individuals to definitively arrange 

 the whole of them under their appropriate heads. 



Vak. a. (the traditional alhula of Adams.) Sohd, abbre- 

 viated, uniform white ; ribs regular, almost per- 

 pendicular, with spirally striated intervals ; lip 

 usually marginated (pi. LXXVI. f. 8). 



Vau. B. {typical or first-named) ovate conic ; pale fulvous, 

 with darker spots, and a purple apex ; numerous 

 very fine longitudinal folds, that are often partially 

 reticulated by obscure spiral lines ; pillar often 

 rufous (pi. LXXXII. f. 5, 6). 



Var. C. (tenuis). Produced, thin, either spotted or streaked 

 with coloured lines that run down from the upper 



