42 
LAND AND FRESH-WATER MOLLUSKS. 
Pisidium pusillum —(the Dwarf Pisidium ) (PL 
IV., fig. 23).—The yellowish-white shell is oval, 
less compressed than the last, very finely but irre¬ 
gularly striated; the umbones nearly central; the 
posterior a very little more produced than the 
rounder anterior side. It is the least inequi¬ 
lateral of the Pisidia. It is the most common of 
the genus; is universally distributed throughout 
the country,inhabiting ponds, drains, ditches, &c. 
It does not apparently require a constant supply 
of water, being often found in marshy spots in 
company with and adhering to the same stones 
as the land mollusca inhabiting such places. 
Mr. Thompson procured it in the North and 
South of Ireland, among moss which was kept 
moist only by the spray of waterfalls. It is found 
in the newer Tertiary deposits. 
Pisidium obtusale (PI. IV., fig. 20).—The shell 
of this species is closely allied to that of P.pusillum 
in its blunt, nearly central umbones, but is smaller 
and much more ventricose; the umbones are 
rather prominent and very blunt. It is one of 
the rarest of the genus, but occurs throughout 
Great Britain. Mr. J. Jones took it in immense 
numbers from a shallow pond on the hill above 
Mitcheldean, in the direction of the Wilderness, 
of which it appeared to be the only molluscous 
inhabitant. 
Oorbicula consobrina, closely allied to the 
