HKB
Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus.
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.
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.)
The Charæ inhabit the bottom of lakes and ponds, and flourish mostly where the water is charged with carbonate of lime. Their seed-vessels are covered with a very tough integument, capable of resisting decomposition; to which circumstance we may attribute their abundance in a fossil state. The annexed figure (Fig. 47) represents a branch of one of many new species found by Professor Amici in the lakes of Northern Italy. The seed-vessel in this plant is more globular than in the British Charæ,) and therefore more nearly resembles in form the extinct fossil species found in England, France, and other countries. The stems, as well as the seed-vessels, of these plants occur both in modern shell-marl and in ancient fresh-water formations. They are generally composed of a large central tube surrounded by smaller ones; the whole stem being divided at certain intervals by transverse partitions or joints. (See b, Fig. 46.)
Fig. 46: Chara medicaginula. Fig. 47: Chara elastica.
It is not uncommon to meet with layers of vegetable matter, impressions of leaves, and branches of trees, in strata containing fresh-water shells; and we also find occasionally the teeth and bones of land quadrupeds, of species now unknown. The manner in which such remains are occasionally carried by rivers into lakes, especially during floods, has been fully treated of in the “Principles of Geology.”