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.
Ampullaria (Fig. 43) is another genus of shells inhabiting rivers and ponds in hot countries. Many fossil species formerly referred to this genus, and which have been met with chiefly in marine formations, are now considered by conchologists to belong to Natica and other marine genera.
All univalve shells of land and fresh-water species, with the exception of Melanopsis (Fig. 34), and Achatina, which has a slight indentation, have entire mouths; and this circumstance may often serve as a convenient rule for distinguishing fresh-water from marine strata; since, if any univalves occur of which the mouths are not entire, we may presume that the formation is marine. The aperture is said to be entire in such shells as the fresh-water Ampullaria and the land-shells (Figs 38-42), when its outline is not interrupted by an indentation or notch, such as that seen at b in Ancillaria (Fig. 45); or is not prolonged into a canal, as that seen at a in Pleurotoma (Fig. 44).
The mouths of a large proportion of the marine univalves have these notches or canals, and almost all species are carnivorous; whereas nearly all testacea having entire mouths are plant-eaters, whether the species be marine, fresh-water, or terrestrial.
There is, however, one genus which affords an occasional exception to one of the above rules. The Potamides (Fig. 37), a subgenus of Cerithium, although provided with a short canal, comprises some species which inhabit salt, others brackish, and others fresh-water, and they are said to be all plant-eaters.
Among the fossils very common in fresh-water deposits are the shells of Cypris, a minute bivalve crustaceous animal.[[3]] Many minute living species of this genus swarm in lakes and stagnant pools in Great Britain; but their shells are not, if considered separately, conclusive as to the fresh-water origin of a deposit, because the majority of species in another kindred genus of the same order, the Cytherina of Lamarck, inhabit salt-water; and, although the animal differs slightly, the shell is scarcely distinguishable from that of the Cypris.