PISID1A. 197 



distinguishing species so nearly allied. He says : 

 "Size, substance, sculpture, and lustre are not of much 

 account, as they mainly depend on the chemical ingre- 

 dients of the water inhabited by these molluscs as well 

 as on their supply of food." He says that he has to 

 the best of his ability worked out the subject of spe- 

 cies, and submitted his labours to the criticism of 

 naturalists. As an exemplification of his labours he 

 states that his "cabinet contains no less than 274 

 parcels of Pisidia collected from various sources 

 during thirty or forty years. Among the British and 

 Continental specimens he has examined he can only 

 recognise six distinct species. The British Pisidia 

 are five in number, distinguishable mainly by their 

 forms, i. P. amnicum ; 2. P. fontinale, triangular; 3. 

 P.pusillum, oval ; 4. P. nitidum, round; 5. P. roseum, 

 oblong. The body of amnicum is grayish-white, 

 rather transparent, tube short, sub-conical, obliquely 

 truncate at its orifice, foot broad at its base, abruptly 

 pointed, and very extensible, mantle bordered with 

 grey. The shell solid and glossy, horn-coloured, length 

 0*3, breadth 0*375. Tne epidermis is thick, beaks 

 prominent, ligament short, inside blueish-while and 

 nacreous. It is the largest kind of Pisidium. The 

 epidermis of fontinale is thin, beaks acute, inside 

 white. In many respects similar to preceding species. 

 There are several varieties of both. They inhabit 

 slow streams, ponds, and canals in all parts of the 

 kingdom. 



The body Qipusillum is whitish, with occasionally a 

 tinge of yellow or red, the foot is longer than the shell, 

 the mantle bordered with reddish-gray. The shell is 



