Ch. in.] 



FROM MARINE FORMATIONS. 



31 



such as that seen at 5 in Ancillaria (fig. 52) ; or is not prolonged into 

 a canal, as that seen at a in Pleurotoma (fig. 51). 



The mouths of a large proportion of the marine univalves have these 

 notches or canals, and almost all such species are carnivorous ; whereas 



Fig. 51. 



Pleurotoma 



rotata. 



Subap. hills, 



Italy. 



Ancillaria subulata. Barton clay. 



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.* 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 Cytkerina of Lamarck (see above, fig. 21, p. 26), in- 

 habit 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 Charce 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 Charce, and therefore more nearly resembles in form the ex- 

 tinct fossil species found in England, France, and other countries. The 



* For figures of fossil species of Purbeck, see below, ch. xx. 



