We may distinguish a freshwater formation, first, by the absence of many fossils almost invariably met with in marine strata. For example, there are no sea-urchins, no corals, and scarcely any zoophytes; no chambered shells, such as the nautilus, nor microscopic Foraminifera. But it is chiefly by attending to the forms of the mollusca that we are guided in determining the point in question. In a freshwater deposit, the number of individual shells is often as great, if not greater, than in a marine stratum; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent freshwater and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Blainville's system, excluding those of extinct species and those without shells, amount to about 200 in number, of which the terrestrial and freshwater genera scarcely form more than a sixth.[28-A] [Illustration: Fig. 25. _Cyclas obovata_; fossil. Hants.] [Illustration: Fig. 26. _Cyrena consobrina_; fossil. Grays, Essex.] [Illustration: Fig. 27. _Anodonta Cordierii_; fossil. Paris.] [Illustration: Fig. 28. _Anodonta latimarginatus_; recent. Bahia.] [Illustration: 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. [Illustration: Fig. 30. _Gryphaea 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. [Illustration: Fig. 31. _Planorbis euomphalus_; fossil. Isle of Wight.] [Illustration: Fig. 32. _Lymnea longiscata_; fossil. Hants.] [Illustration: Fig. 33. _Paludina lenta_; fossil. Hants.] The univalve shells most characteristic of freshwater deposits are, _Planorbis_, _Lymnea_, and _Paludina_. (See figures.) But to these are occasionally added _Physa_, _Succinea_, _Ancylus_, _Valvata_, _Melanopsis_, _Melania_, and _Neritina_. (See figures.) [Illustration: Fig. 34. _Succinea amphibia_; fossil. Loess, Rhine.] [Illustration: Fig. 35. _Ancylus elegans_; fossil. Hants.] [Illustration: Fig. 36. _Valvata_; fossil. Grays, Essex.] [Illustration: Fig. 37. _Physa hypnorum_; recent.] [Illustration: Fig. 38. _Auricula_; recent. Ava.] [Illustration: Fig. 39. _Melania inquinata._ Paris Basin.]