68 PALUDINID^E. 



floating with tolerable rapidity along the under surface 

 of the water. The shell is often encrusted with a mineral 

 deposit or covered with an algoid or confervoid growth. 

 The variety 5 resembles the Cydostoma vitreum of Dra- 

 parnaud and the Paludina diaphana of Michaud. Some- 

 times the shell is distorted by having the upper part of 

 the spire twisted to one side, or by the last whorl having 

 a few obscure spiral ridges. 



This small, but abundant, species has received a great 

 number of names from modern conchologists, in conse- 

 quence of their referring it to ill-defined species of ancient 

 authors. I do not believe that it is the Turbo stagnorum 

 of Baster, because he describes the habitat to be " in 

 aquis dulcibus," and the aperture or mouth to be mar- 

 gined. Nor can I identify it with the Helix octona of 

 Linne", which is said to have eight whorls and a round 

 aperture. ISTor is it, in my opinion, the Turbo thermalis 

 of Gmelin, because he gives an inland habitat (Pisa) and 

 says that the shell is white and has only four whorls. 

 There can, however, be no doubt of its being the Cy- 

 clostoma acutum of Draparnaud ; and this specific name 

 has been adopted by almost every continental naturalist. 

 The Paludina muriatica of Lamarck is evidently H. ulvce. 



This species differs from H. similis in its long spire, 

 the suture not being channeled, and in the umbilical 

 chink being very much smaller. From H. ulvce it may 

 be known by its being less than half the size of that 

 species, its much deeper suture, the body or last whorl 

 not being keeled (as is the case in H. ulvm\ as well as in 

 the inner lip being disconnected from the columella. 



The shells which Mr. Pickering found some years ago, 

 about two miles below Gravesend, together with a spe- 

 cimen of Litiopa bombyx, and which Forbes and Hanley 

 considered (but with some doubt) to be a variety of 



