PART I. CHAPTER III. 



47 



Distinction of Freshwater from Marine Formations. 



Fig. 47. 



Fig. 48. 



Pleurotomata 



rotata. 



Subap. hills, 



Italy. 



Jlncillaria sululata. London clay. 



have either these notches or canals, and all these species are, 

 without exception, carnivorous ; whereas nearly all testacea hav- 

 ing 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. 42.,) 

 although provided with a short canal, comprises some species 

 which inhabit salt, others brackish, and others fresh water. 



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 

 another kindred genus of the same order, the Cytherina of 

 Fig. 49. Fig. 50. 



Chara medicaginula ; Chara elastica ; recent. Italy. 



fossil. Isle of Wight, a, Sessile seed-vessel between the division of the 



a, Seed-vesels, leaves of the female plant. 



magnified 20 b, Transverse section of a branch, with five seed- 

 diameters, vessels magnified, seen from below upwards. 



6, Stem magnified. 



* See figures in chap, on Wealden, Port II. 



