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. 28.

Anodonta latimarginatus; recent. Bahia.

Fig. 29.

Unio littoralis; recent. Auvergne.

Almost all bivalve shells, or those of acephalous mollusca, are marine, about ten only out of ninety genera being freshwater. Among these last, the four most common forms, both recent and fossil, are Cyclas, Cyrena, Unio, and Anodonta (see figures); the two first and two last of which are so nearly allied as to pass into each other.