A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Fig. 30.

Gryphæa incurva, Sow. (G. arcuata, Lam.) upper valve. Lias.

Lamarck divided the bivalve mollusca into the Dimyary, or those having two large muscular impressions in each valve, as a b in the Cyclas, [fig. 25.], and the Monomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen in [fig. 30.] Now, as none of these last, or the unimuscular bivalves, are freshwater, we may at once presume a deposit in which we find any of them to be marine.

Fig. 31.

Planorbis euomphalus; fossil. Isle of Wight.

Fig. 32.