16 SPH EHRIDA. 
«A, Triangular (P. amnicum, P. fontinale). 
B. Oval (P. pusillum). 
C. Round (P. nitidum). 
D. Oblong (P. roseum).” 
The Pisidia differ from the Spheria in having a 
single siphon, while the Spheria 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 Spherium. 
Young specimens need not be mistaken for P. fontinale, 
as the latter shell is not so deeply groved, and far more 
ventricose. ri 
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. 
