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 _Charae_ 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 _Charae_, 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.) [Illustration: Fig. 53. _Chara medicaginula_; fossil. Isle of Wight. _a._ Seed-vessel. magnified 20 diameters. _b._ Stem, magnified.] [Illustration: Fig. 54. _Chara elastica_; recent. Italy. _a._ Sessile seed vessel between the division of the leaves of the female plant. _b._ Transverse section of a branch, with five seed-vessels magnified, seen from below upwards.]