16 SPHJ3RIDJE. 



"A. Triangular (P. amnicum, P. fontinale). 



B. Oval (P. pusillum). 



C. Round (P. nitidum). 



D. Oblong (P. roseum)." 



The Pisidia differ from the Spharia in having a 

 single siphon, while the Sphceria have two. 



1. P. AMNICUM (inhabiting rivers). 



Triangular, tumid, solid, glossy, quite opaque when 

 adult, having deep concentric ridges, greyish horn-colour ; 

 teeth (as are those of "all the Pisidia) the same as those of 

 the last genus. 



P. amnicum is found in ponds, canals, rivers and 

 lakes throughout the country. There is no mistaking 

 this species when adult on account of its much greater 

 size than any of the others ; its triangular shape dis- 

 tinguishes it from the members of the genus Spharium. 

 Young specimens need not be mistaken for P. fontinale, 

 as the latter shell is not so deeply groved, and far more 

 ventricose. 



2. P. FONTINALE (inhabiting springs). 



Triangular ; so tumid that it has a cuboid appearance ; 

 thin, greyish horn-colour ; more transparent than P. am- 

 nicum; beaks prominent ; scars deep. 



Found in sluggish streams, canals, ponds, and ditches 

 throughout the country. 



