Almost all bivalve shells, or those of acephalous mollusca, are marine, about sixteen only out of 140 genera being fresh-water. 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.
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. 18, and Unio, Fig. 22, and the Monomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen in Fig. 23. Now, as none of these last, or the unimuscular bivalves, are fresh-water,[[2]] we may at once presume a deposit containing any of them to be marine.
The univalve shells most characteristic of fresh-water deposits are, Planorbis, Limnæa, and Paludina. (See Figures.) But to these are occasionally added Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, Potamides, and Neritina (see Figures), the four last being usually found in estuaries.
Some naturalists include Neritina (Fig. 35) and the marine Nerita (Fig. 36) in the same genus, it being scarcely possible to distinguish the two by good generic characters. But, as a general rule, the fluviatile species are smaller, smoother, and more globular than the marine; and they have never, like the Neritæ, the inner margin of the outer lip toothed or crenulated. (See Fig. 36.)