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THE AMERICAN NATURALIST [Vol. XLII 



ish waters are smaller than those in normal sea water. 

 Cardium edule is found in the British Isles in harbors and 

 high up tidal rivers, where the water gets brackish; its 

 shell is modified, invariably reduced in size, thin, and with 

 less strongly marked external characters. The ten 

 species of Cardium in the Caspian Sea are all aberrant 

 forms, all related back to C. edule, small, thin and smooth, 

 with lateral or central teeth or both suppressed. So like- 

 wise with the cockles of the Black and Baltic Seas ; in the 

 latter the salinity is reduced one half by the water from 

 the rivers. 2 The Greenland cockle lives in estuaries ; it is 

 no longer found in Europe but is very abundant in the 

 Pliocene (Crag) of Suffolk and Norfolk, especially in the 

 fluvio-marine portions. It is thin, smooth, almost edent- 

 ulous, with rudiments of a single tooth in each valve in the 

 young shells which finally disappear. 3 



Some forms, as Scrobicularia and Mactra solida, have 

 become thoroughly adapted to a brackish water environ- 

 ment and attain their largest size there. But many, if not 

 most species, which live in normal sea water and in 

 brackish water are smaller in the latter, as is true of 

 Cardium edule, Mya arenaria and Littorina littorea. 



(b) Change due to a Concentration of Salt, Iron, etc.— 

 When a body of water has become concentrated to a point 

 where precipitation of its salt takes place, as is practically 

 the case in the Great Salt Lake or entirely so in the Dead 

 Sea, no life can exist in it. But from the normal sea 

 water to this condition there takes place progressively a 

 lessening both in the number of species and in the size of 

 the individuals there present. 



Many fossil dwarf faunas have been ascribed to this 

 cause, as, for example, those of the Permian. 



That even a comparatively slight concentration of the 

 sea water may produce a dwarfing in its fauna appears to 

 be indicated by the western Mediterranean species- 

 Dana gives the amount of saline matter in the Mediter- 

 ranean as 3.9 per cent, as against 3.6 per cent, for the 



