38 
LAND AND ERESH-WATER MOLLUSKS. 
Genus Pisidium. 
This genus is separated from Gy das to receive 
the smaller species,, which have inequilateral 
shells, and have only one siphonal tube; the 
incurrent aperture being confluent with that for 
the passage of the foot (see fig. 3, p. 32). They 
have much the same habits as Gy das, and live 
in similar situations. It is very difficult to dis¬ 
tinguish the species from each other, which, 
therefore, necessitates detailed descriptions of 
each. The largest and one easily recognized is— 
Pisidium amnicum —■ ('inhabiting rivers) (Pl. 
III., fig. 11).—The shell is triangular, deeply 
grooved concentrically, of a whitish-grey or pale 
brown; umbones very little produced; four lines 
long, one and a half thick, and three wide. It 
usually buries itself in the mud, but climbs by 
the aid of its long flexible foot among the aquatic 
plants. It is common, and universally distributed 
over Great Britain and Europe. It is fossilized 
in the newer Tertiaries. 
Pisidium cinereum (PL IV., fig. 21) approaches 
the last, as regards size, more than any of the 
other species. Shell greyish, with one or two 
broad bands, more compressed and oval than the 
others of the genus, finely striated; umbones 
obtuse and prominent, sometimes slightly capped. 
