50 DREISSENID^. 



polymorpha is enumerated as one of the fossils. Mr. 

 Prestwich informs me that this deposit was in all pro- 

 bability contemporaneous with those of St. Acheul and 

 Amiens, and that at all events it belongs to what is 

 termed by modern geologists the upper tertiary forma- 

 tion. I am therefore not without hope that this remark- 

 able shell may be discovered in the corresponding strata 

 in this country. It is frequently found, in a recent or 

 living state, with the Anacharis alsinastrum, an aquatic 

 plant which chokes up our canals and is said to have been 

 imported from North America. Respecting the Anacharis, 

 Messrs. Hooker and Arnott, in their excellent work on 

 the British Flora, remark that "it seems inexplicable how 

 this plant should have occurred in so many different 

 places at the same time." Perhaps if the Eriocaulon 

 septangulay^e, or Naias flexilis, both of which are also 

 North-American water-plants, and are at present con- 

 fined to a very few stations in the Hebrides and West of 

 Ireland, had been placed in conditions which were more 

 favourable to their growth and propagation, each of them 

 might have spread with as great rapidity as the Ana- 

 charis. If, as I believe, the indigenousness of the 

 Dreissena as regards this country should hereafter be 

 established, the ingenious theories which have been pro- 

 posed to account for the mode of its transport across the 

 seas will not require further discussion. 



J 



