PISIDIUM, jaye 
Genus.—Pisipium C. Pfeiffer. 
Shell inequilateral ; deaks near the shorter or posterior end ; azzmal 
with only one siphon. 
These little shells are usually a great trouble to the 
collector on account of their variability and similarity, 
and conchologists have differed greatly as to the number 
of species. The thanks of all collectors, therefore, are 
due to Dr. Jeffreys, who has reduced the number to five 
fairly well-marked species, distinguishing them as follows :— 
a. Triangular. (P. amnicum, P. fontinale). 
b. Oval. (P. pustllum). ° 
c. Round. (LP. xitidum). 
d. Oblong. (P. milium). 
The table on the opposite page may be a help to identification. 
I. P. AMNICUM (2xhabiting rivers) Miiller. Vl. IX., f. 5 
Triangular, tumid, solid, glossy, quite opaque when adult, having 
deep concentric ridges, greyish horn-colour ; teeth (as those of all the 
Pisidia) the same as those of the last genus, but proportionately 
more strongly developed. L. 75mm. B. 93 mm. 
P. ammnicum is found in ponds, canals, rivers, and lakes 
in all parts of Great Britain as far north as the Forth, and 
in Ireland rarely in the south and east. 
There is no mistaking it when adult for any others of 
the genus on account of its much greater size, and its 
triangular shape distinguishes it from the members of the 
genus Spherium. Young specimens need not be mis- 
