The mouths of a large proportion of the marine univalves have these notches or canals, and almost all such species are carnivorous; whereas nearly all testacea having entire mouths, are plant-eaters; whether the species be marine, freshwater, or terrestrial.

There is, however, one genus which affords an occasional exception to one of the above rules. The Cerithium ([fig. 44.]), 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 freshwater deposits are the shells of Cypris, a minute crustaceous animal, having a shell much resembling that of the bivalve mollusca.[31-A] 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 freshwater origin of a deposit, because the majority of species in another kindred genus of the same order, the Cytherina of Lamarck (see above, [fig. 21.] [p. 26.]), inhabit salt water; and, although the animal differs slightly, the shell is scarcely distinguishable from that of the Cypris.

The seed-vessels and stems of Chara, a genus of aquatic plants, are very frequent in freshwater strata. These seed-vessels were called, before their true nature was known, gyrogonites, and were supposed to be foraminiferous shells. (See [fig. 53. 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. 54.]) 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 freshwater formations. They are generally composed of a large tube surrounded by smaller tubes; the whole stem being divided at certain intervals by transverse partitions or joints. (See b, [fig. 53.])

Fig. 53.

Chara medicaginula; fossil. Isle of Wight.