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By J. 0. MANSEL-PLEYDELL, P.L.S , F.G.S. 



PREFACE. 



HE origin of the distribution of Marine mollusca 

 can be accounted for far more easily than that 

 of land and freshwater, as their young, both of 

 univalves and bivalves have swimming powers, 

 and after being drifted about by the tides and 

 currents, can settle down, far away from their 

 original homes ; thus the byssus-spinning mussel and pinna, the 

 quasi-sedentary cockle or scollop, and the slow-creeping gas- 

 teropod can be conveyed in their juvenile state to distant localities, 

 favourable to adult life. The distribution of freshwater 

 mollusca cannot be thus accounted for, since migration is 

 debarred them as soon as the margin of the pond or lake is 

 reached or the stream in which they sport has joined the sea, 

 wading birds whose flights are long and distant can, with ease, 

 transport from one place to another a mollusc, either in its egg 

 or mature state, which has adhered to their feet or legs 



