FRESH-WATER AND MARINE FORMATIONS, 
57 
Fig. 44. 
Fig. 45, 
Pleurotoma exorta, Brand. 
Upper and Middle Eocene. 
Ancillaria suhulata, Sow. 
Barton clay. Eocene. 
Barton and Bracklesham. 
The mouths of a large proportion of the marine univalves 
have these notches or canals, and almost all species are car¬ 
nivorous; 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 occasion¬ 
al 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 ani¬ 
mal.'^ 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 Gytherma of 
Lamarck, inhabit salt-water; and, although the animal dif¬ 
fers 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 Charm inhabit the bottom of lakes and ponds, and 
flourish mostly where the water is charged with carbonate 
of lime. Their seed-vessels are covei*ed with a very tough 
integument, capable of resisting decomposition; to which 
circumstance we may attribute their abundance in a fossil 
* For figures of fossil species of Purbeck, see below, Chap. XIX. 
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