PISIDIUM. 7 



GENUS IL PISIDIUM* C. PFEIFFER. 



Body, sides unequal ; mantle with only one siphon. 

 Shell, sides unequal ; beaks near the anterior end. 



The members of this genus differ from those of the 

 preceding one (Sphceriuni), in having only one siphon, 

 and in their shells being less equilateral and very 

 much smaller; in other respects, however, they re- 

 semble them, their habits and homes being almost 

 identical ; they possess, too, in common with them 

 the power of creeping in an inverted position on the 

 under surface of the water. They occur in vast 

 numbers in rivers, lakes, ponds, and stagnant water, 

 and are a favourite food of fish and ducks. The shell 

 is very often thickly coated with a deposit from the 

 water in which they live. 



Five species only, of this genus, are as yet known 

 to occur in Great Britain, and as they are not very 

 easily identified, I cannot do better than follow the 

 lead of the gifted author of ' British Conchology,' and 

 " divide them according to their shape, as follows : 



"A. Triangular. I. P. amnicum, 2. P. fontinale. 



"B. Oval 3. P. pusillum. 



" C. Round. 4. P. nitidum. 



" D. Oblong. 5. P. roseum." B.C, vol. i. p. 19. 



A. Triangular. 

 I. PISIDIUM AM'NICUM,t MtJLLER. PL. I. 



Body whitish or greyish, slightly transparent ; foot rather long, 

 broadish at the base, abruptly pointed, capable of great exten- 

 sion ; mantle bordered with grey ; siphon short, somewhat 

 conical, obliquely truncated at its extremity. 



Pea-shaped. f Inhabiting rivers. 



