spire-shells and Marsh-snails 



233 



opalina on Corallina officinalis and Laniinaria ; 

 J. cjlohularis on Laminaria at Croulin Island, 

 Skye. 



In the genus Bithynia again we come upon two 

 fresh-water species that have obviously been derived 

 from marine forms. The name is badly chosen, for it 

 might indicate that the species inhabit deep water, 

 whereas streams, ponds, and ditches are their usual 

 habitat. The figure represents the Common Bithynia 

 (B. tentaculata) of the natural size. The pale brown 

 shell is a symmetrical spiral of conic-oval outline, 

 fairly solid ; the oval mouth closed by a thick shelly 

 operculum ; umbilicus a narrow chink. The animal 

 is dark brown or black above, paling to a 

 dirty grey below, with a long deeply cleft 

 snout, one branchial plume, and thread-like 

 diverging tentacles. The prominent black 

 eyes are not stalked. Leach's Bithynia (B. 

 leachii) is a much smaller species (a quarter 

 of an inch long), with a thinner, semi- 

 transparent shell, swollen whorls, and a more distinct 

 umbilicus. The mouth of the shell is nearly- round, 

 and the operculum almost circular. Both species 

 frequent sluggish rivers and still waters, but whereas 

 the larger form is widely distributed everywhere in 

 England, Ireland, and Wales, the smaller is more 

 local and less abundant. They feed 

 indifterently on animal and vegetable 

 substances ; and lay their eggs in bands 

 on stones and water-plants. 



The Sentinel-shells (Assmiinea) have 

 been so-called because their eyes are borne upon long 

 stalks which give them the power of elevation 



Dun Sentinel 



