FOSSIL FRESH-WATER BIVALVES. 413 
for paving, or other economical purposes. Very sharp casts 
may be obtained from this rock by merely breaking the 
stone to pieces. In the Whetstone of Blackdown, Devon, 
beautiful silicified Trigonize are occasionally found. Tisbury, 
in Wiltshire, yields very fine specimens, and in some exam- 
ples, Mr. G. B. Sowerby has detected remains of the 
ligament. 
FOSSIL FRESH-WATER BIVALVES. 
The animals of the shells hitherto described are, with 
scarcely any exception, inhabitants of the sea; and the 
marine origin of the strata in which they occur, may conse- 
quently be inferred, with but little probability of error. I 
now propose noticing the fossil remains of those bivalves 
which inhabit rivers, lakes, streams, and pools of fresh 
water. The marine, or fresh-water, character of fossil shells, 
is inferred from their resemblance to the recent mollusca, 
whose habits are known; for the shells alone present no un- 
equivocal marks, by which even the experienced conchologist 
can pronounce whether an extinct form belonged to a marine 
or to a fluviatile mollusk, although certain characters may 
admit of an approximative inference. Thus, for instance, as 
none of the known living fresh-water bivalves belong to the 
previous division, the Monomyaria, the presence in a stra- 
tum of numerous shells with but one muscular impression, 
would afford a fair presumption of the marine origin of such, 
deposit. The remains with which the shells are associated 
and the mineralogical characters of the strata in which they 
occur, would, of course, afford important corroborative 
evidence.* 
The living fresh-water bivalves comprise but a few genera 
and species; and those which have been found fossil in the 
British strata belong to but four or five genera. Their dis- 
* See Sir C. Lyell on the distinction between fresh-water and 
marine deposits. Ly. p. 27, et seg. 
