PISIDIUM. 17 



Yar. I. Henslowana. The valves have a plate-like 

 appendage near the beaks. 



Yar. II. pulchella. Strongly grooved, glossy. 



Yar. III. pallida. More tumid, paler, irregularly 

 striate. 



Yar. IY. cinerea. Larger and flatter. 



3. P. PUSILLUM (small). 



Oval, thin, moderately glossy, finely striate concen- 

 trically, yellowish white ; beaks nearly central ; ligament 

 inconspicuous. 



Common in ditches and swamps. Its oval form, as 

 well as its blunter and more central beaks, distinguish 

 it from the last species. 



Yar. I. obtusalis. Smaller and more tumid. 



Yar. II. grandis. Much larger (near Manchester). 



4 P. NITIDUM (shining). 



Roundish, tumid above, compressed below, very glossy ; 

 scars distinct. 



It is found in ponds throughout the country. Its 

 outline is rounder and its epidermis more glossy than 

 the rest of the Pisidia. Its siphon is funnel-shaped. 



Yar. I. splendens. Lemon-coloured. 



Yar. II. globosa. Spheroidal in form. 



