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NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 1 7 



Family DREISSENID^. 



Genus DREISSENA Van Beneden. 



Mytilomia, Cantr. Myoconcha, D'Orb. 



Caslogonia, Bronn. Congeria, Partsch. 



Dithalmia, Say. Enocephalus, Munst. 



The species of this genus are widely distributed, being 

 found in Europe, Asia, Africa, North and Central America, and 

 West Indies, and Mr. Garrett has recently discovered a species 

 in Viti, not distinguishable from D. Africance. 



The genus is subdivided by H. and A. Adams into Drei'ss- 

 ena (sensu stricto), of which D, polyviorpha is the type, and 

 Praxis Adams, a group characterized by a small lamina affixed 

 to the septum. 



At the time of publication of Adams' " Genera of Recent 

 Mollusca" fifteen species of recent, and about fourteen fossil 

 forms were known. Several species of Dreissena inhabited 

 the fresh waters of the Isle of Wight during the Upper Eocene 

 epoch. 



Dreissena polymorpha (Pallas). 



Locally abundant. 



In Yorkshire this shell is found exclusively in or near the 

 canals or navigable rivers which intersect the manufacturing 

 districts of the West Riding. 



We have various records of unsuccessful attempts to intro- 

 duce it into new localities. Mr. R. J. Bell stated in 1843 that 

 Mr, Bean introduced it at Scarborough a few years before that 

 date, but was of opinion that they all subsequently perished. 

 Mr. G. F. Linney says " it was introduced into the Ackworth 



School Canal but did not flourish." 



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