483 



epidermis, and marked with a few circular wrinkles : 

 vertex (liarp, placed near to one end, reflexed, or fomc- 

 what hooked : aperture or opening oval, margin thin^ 

 infide glofTy. Length three-eighths of an inch ; breadth 

 two-eighths ; height not quite fo much. 



A variety is fometimes met with, finely ftriated longi- 

 tudinally from the beak; and we received fome fpeci- 

 mens from a frefh-water ftream, near Folkjlonc in Kent^ 

 more fl;-ongly flriated than ufual ; which were confider- 

 ed as a diflinfl fpecies. 



Thefe were of an inferi6r fize, and covered v/ith 

 a duflcy epidermis: they appear to us to be only a 

 variety, differing in nothing but tlxe flrise being more 

 confpicuous. 



ThisTpecles has generally been confidered as the P. la- 

 czi/2mof LiNN^us, thefynonyms, however, of Gmelix, 

 are certainly thofe of the P.Jluviaiilis. It is a very com- 

 mon fliell in moft of our rivers, and in many fmall 

 ftreams and flagnant waters, adhering to flones, and 

 rarely to the leaves or ftalks of aquatic plants. Is al- 

 moft the only fhell, we believe, found to inhabit the rapid 

 torrents, with the Mya margaritifera. In DevonJIiirc, 

 efpecially in the fouth, every little ffream produces abun- 

 dance ; even the cold rills that originate from fprings, 

 and flow down the wooded declivities, except where gra- 

 nite prevails : on the contrary, about the eaflern parts 



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