FOSSIL FRESH-WATER UNIVALVFS. 471 
the deposit having been formed in shallow, or in deep water. 
Thus when simple univalves largely predominate under 
circumstances that indicate they were imbedded in their 
native habitats, it may be safely concluded that the rock is 
of littoral formation; or, in other words, was deposited in 
shallow water, near the sea-shore; and, on the contrary, 
when Nautili, Ammonites, and the shells of other mollusca 
known to live in deep waters abound in a formation, it may 
be presumed that the strata were formed in the tranquil 
depths of the ocean. The number of described species from 
the British strata is nearly eight hundred; and these are 
distributed throughout the sedimentary formations, from 
the Silurian to the newest Tertiary; the latter containing 
by far the greater proportion. 
FRrEsH-WATER Univatves.—The fossil shells of Gastero- 
poda that are undoubtedly fluviatile, comprise but few 
genera and species, and are confined to those deposits, which, 
from the corroborative proofs afforded by other organic 
remains, are unquestionably of fresh-water origin. Such 
are the intercalated beds of clay and limestone in the 
London and Paris basins, the Wealden formation, and 
certain strata in the Carboniferous system. The most 
numerous specimens are principally referable to the common 
fluviatile genera, Paludina, Limnea, Planorbis, and Mela- 
nopsis (see Ly. p. 29). 
Patupina. Lign. 133, fig.1. (Wond. p. 401, Ly. p. 29.) 
—This common river shell is of a conoidal form, and the 
whorls of the spire, and the aperture, are rounded. Eleven 
British species are known. In the tertiary fresh-water beds 
of Headon Hill, at Alum Bay, Paludinz with the shells 
perfect, and of a dull white colour, are abundant; and also 
in the limestone at Shalcombe, in the Isle of Wight, in the 
state of casts. In both these localities the Paludine are 
associated with other fresh-water shells. But the grand 
