Fig. 43: Ampullaria glauca. 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). Fig. 44: Pleurotoma exorta. Fig. 45: Ancillaria subulata. 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. Fresh-water Fossil Plants.—The seed-vessels and stems of _ Chara,_ a genus of aquatic plants, are very frequent in fresh-water strata. These seed-vessels were called, before their true nature was known, gyrogonites, and were supposed to be foraminiferous shells. (See Fig. 46, _a_.)