1 8 NET.SON AND TAYLOR: ON, YORKSHIRE MOLLUSCA. 



Mr. H. J. Bellars confirms the supposition of Dr. Jeffreys 

 that this species may have existed in other parts of England 

 prior to its discovery in the Commercial Docks in 1824, stating 

 that it " had been observed many years previously in great pro- 

 fusion in Whittlesea Mere." Mr. R. J. Bell (Zool., 1843, 

 p. 283) records the great abundance of this species in the docks 

 at Goole, three or four years after their opening in 1838 ; and 

 judged from the numbers of dead ones around that it was evident 

 many generations had passed away. Mr. Bell also remarks 

 that as far as the influence of the salt water extends into the 

 dock not one solitary individual can be found, the line of de- 

 marcation being as perfect as the joint between one stone and 

 another in a wall. 



Mr. R. Tate says that it has a predilection for shady 

 places, abounding beneath the shade of bridges, and in docks 

 attaching itself to the under surfaces of floating timber. In its 

 young state it frequently attaches itself to the vegetation just 

 beneath the surface. It forms a frequent food of the water-rats, 

 who leave the empty shells in heaps on the banks. 



D. polymorpha is absent from the newer tertiaries of this 

 country, but is found in those of Transylvania and Moravia, 

 and near Vienna. 



Mr. Bellars, of Chester, has noticed and described three 

 varieties, and continental conchologists distinguish several more. 

 A very curious malformation has been found at Nostel by Mr. 

 Conacher and others, in which the periodic growths have a very 

 distinct step-like appearance. Pearly concretions are not unfre- 

 quently found in this species. 



3. Vale of York — Formerly found in the artificial pond in the York 



Museum Gardens, where it had doubtless been introduced, R. B. 

 Cook. 



4. Number — Profusely abundant in the Docks at Goole, R. J. Bell, 1843. 

 6. Trent — River Don, at Conisbro', in 1836, where it had evidently 



existed for some time, R. J. Bell (Zool., 1843). 



Trans.Y.N.U., 1880 (pub. 1883). Series C 



