4;d 



for other authors have confidered feme varieties of this 

 fhell as diftinft fpecies: the P. caerulea o{ Gmelin is 

 certainly nothing elfe than a full grown fiiell. 



It is remarkable, however, that the very thin tranf- 

 parent (hells have the bottom, or margin, always even, 

 and not turned upwards at the end ; and yet feme, not 

 half the fize, are more opaque, and have all the fliape of 

 full grown ones. 



It has alfo been remarked, that the pellucid variety is 

 always found on the leaves, and the others on the flalks 

 of algce ; however this may be, there appears no pofTi- 

 bility of fixing a permanent diRin6lion.* 



It is a common fpecles on many parts of the Cornifti 

 and Devonjhire coads : is never found adhering to rocks, 

 but we have frequently taken it alive after ftorms, ad- 

 hering to the Fucus digitatus, that has been thrown 

 up by the violent agitation of the fea. It is not uncom- 

 mon on the coafl of Dorfefjliire ; and we have received 

 it from SandzL'ich in Kent, where it is alu'ays found on 

 the fame plant ; the ftalk of which the animal excavates, 

 probably as food, and forms a cell ; fometimes two or 

 three are found in the fame: and Mr. Boys aflures us, 



it 



* Variety of fituation will occafion variety in fhape ; ■while young it feeds 

 only on the leaves, and their fhape at the margin correfponds with the flat 

 furface they adhere to: fo when they attack the ftalk, tliey acquire that 

 fhape which a convex furface requires for clofe contaS: ; and which occafions 

 .tfie difference of growth in the old and young (hells. 



