NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA. 37 



Planorbis lineatus Walker. 



Abundant, but extremely local. 



So far as is at present known, this species in Yorkshire 

 only occurs in one or two localities round York and in one in 

 Holderness. Askham (or more correctly speaking, Dringhouses) 

 Bog is its great stronghold, and here it is in great abundance. 

 On Strensall Common it occurs but only sparingly. A few are 

 said to have been found in Hobmoor ponds, while in 1855 a 

 few were found in the Tillmire ponds, and in 1859 in the direc- 

 tion of Osbaldwick, all these places being in the immediate 

 vicinity of the city of York. Mr. R. M. Christy, who collected 

 round about York with great assiduity for some years, informs 

 us that he himself has not met with the species elsewhere than 

 at Askham, where in some ponds it abounds, whilst in others 

 there are scarcely any at all. 



In its habitat this species differs from Planorbis nitidus, 

 which prefers living among decaying leaves in stagnant pools ; 

 F. lineatus by preference inhabits clearer and shallower ponds 

 where there is grass growing, and even when its pond is slightly 

 flooded Mr Christy's experience is that it will come up into the 

 flooded grassy margin. 



The Yorkshire localities are interesting on account of being 

 the most northerly habitat in Britain and forming an entirely 

 detached outlier, the nearest known locality being the neigh- 

 bourhood of Norwich. 



Lovell Reeve mentions that it has been found in the south 

 of Scotland, but this observation has not been confirmed. 



A highly polished yellow variety is plentiful in freshwater 

 streams near Melbourne, Australia, according to Mr W. F. 

 Petterd, of Launceston. 



Dr. Jeffreys states that it lays only from three to eight eggs, 

 but we are inclined to think that these are the numbers in a 

 single capsule, of which in all probability several are laid in the 



