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stagnant pools, and marshes. These individuals have been washed away from the alluvial plains of the great river and its tributaries, some from mountainous regions, others from the low country.
Although fresh-water formations are often of great thickness, yet they are usually very limited in area when compared to marine deposits, just as lakes and estuaries are of small dimensions in comparison with seas.
The absence of many fossil forms usually met with in marine strata, affords a useful negative indication of the fresh-water origin of a formation. For example, there are no sea-urchins, no corals, no chambered shells, such as the nautilus, nor microscopic Foraminifera in lacustrine or fluviatile deposits. In distinguishing the latter from formations accumulated in the sea, we are chiefly guided by the forms of the mollusca. In a fresh-water deposit, the number of individual shells is often as great as in a marine stratum, if not greater; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent fresh-water and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Woodward’s system, excluding those altogether extinct and those without shells, amount to 446 in number, of which the terrestrial and fresh-water genera scarcely form more than a fifth.[1]
Fig. 18: Cyrena obovata. Fig. 19: Cyrena fluminatis.
Fig. 20: Anodonta Cordierii. Fig. 21: Anodonta latimarginata. Fig. 22: Unio littoralis.
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.
Fig. 23: Gryphæa incurva.
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.
Fig. 24: Planorbis enomphalus. Fig. 25: Limnæa longiscala. Fig. 26: Pauldina lenta. Fig. 27: Succinea amphibia. Fig. 28: Ancylus velletia. Fig. 29: Valvata piscinalis. Fig. 30: Physa hypnorum. Fig. 31: Auricula. Fig. 32: Melania inquinata. Fig. 33: Physa columnaris. Fig. 34: Melanopsis buccinoidea.
Fig. 35: Neritina globulud. Fig. 36: Nerita granulosa. 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.
Fig. 37: Potamides cinctus.
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.)
The Potamides inhabit the mouths of rivers in warm latitudes, and are distinguishable from the marine Cerithia by their orbicular and multispiral opercula. The genus Auricula (Fig. 31) is amphibious, frequenting swamps and marshes within the influence of the tide.
The terrestrial shells are all univalves. The most important genera among these, both in a recent and fossil state, are * Helix* (Fig. 38), Cyclostoma (Fig. 39), Pupa (Fig. 40), Clausilia (Fig. 41), Bulimus (Fig. 42), * Glandina* and Achatina.
Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus.