40 NELSON AND TAYLOR : ON YORKSHIRE MOLLUSCA, 



Var. albida Nelson. 



20. Lower Tees — A fine specimen in the fish-pond, Acklam, near Middles- 

 brough, in the early part of 1883, with the type, B. Hudson. 



-:-0-:- 



Planorbis nautileus (L.). 



Widely distributed and common. 



This species seems to be" pretty well diffused through the 

 low-lying country in suitable places. It is usually abundant 

 where it occurs, adhering to Anacharis and other plants. 



It is a favourite shell with some kinds of caddis-worms 

 for attaching to their cases, Mr. R. M. Christy has some from 

 Scarborough completely covered by a hundred or more 

 specimens. 



The var. crista is usually found with the type, of which it 

 is generally considered to be merely an immature form. 



Mr. Wilcock, of Wakefield, one of our most assiduous 

 and successful collectors, has found distorted specimens in 

 several localities in the county, but does not specify the precise 

 places. 



3. yale of York — Abundant in some ponds at Askham and Hobmoor, rare 



in the Foss, and very probably in some other places, but its small 

 size renders it difficult of discovery, R.M.C. ; Ouse drift, H. 

 Richardson ; very plentiful along with Spluerium lacustre in a pond 

 near Bootham Stray, York, H. Richardson ; ponds near York, W. 

 C. Hey ; ponds near Tadcaster, F. G. Binnie ; Sherburn, 1870, 

 G. Roberts ; Knaresborough, J.W. ; pond near Jackson's Wood, 

 Wetherby, F. G. Binnie ; pond between Wetherby and Walton, 

 possibly the same place as preceding, J. Emmet ; ditch near Ouse, 

 below York, in 1868, R. Whitwell ! Strensall Common, W. C. 

 Hey. 



4. Number— Fryston Park, H.P. ! Castleford, Geo. Roberts; Milford, 



1875, G. Roberts ; Selby, J.W. ; Drax, J.W. ; Goole, J.W. 



6. Trent — Conisborough, J.W. ; Doncaster, J.W. 



7. Went Vale — P. imbricatiis at Ackworth, not common, found with 



Ancylus lacustris, C.A. (1854) ; Ackworth, extremely abundant in 

 various ponds, C.A. (1874) ; Campsall lake, E. Lankester, 1842, 

 andJ.W., 1875 ; Hemsworth, J.W. ; Went Bridge, J.W. ; Nostel, 



Trans.Y.N.U., 1883 (pub. 18S6). Series C 



