PLEISTOCENE GEOLOGY OF THE THAMES VALLEY. 357 



Planorbis vortex, Linn. One example of this hitherto 

 unrecorded species. It is a rare form in any Pleistocene 

 deposit. 



Planorbis spirorbis, Linn. Several examples. 



Planorbis contortus. Linn. An imperfect example. 

 Recorded by S. V. Wood, but no examples known. 



Planorbis fontanus, Lightfoot. Two examples, one of 

 which has been lost. This is a new record for Grays. It is 

 known from the Pleistocene of Westminster, Clacton, Copford 

 and Barnwell. 



Planorbis lineatus, Walk. Recorded by S. V. Wood ; 

 perhaps an error for P. fontanus. 



Paludestrina ventrosa, Mont. An abundant form. 

 This is the more remarkable as at the present day it is a 

 brackish water form, and there is no other palaeontological 

 evidence of estuarine conditions at Grays. 



Paludestrina marginata, Mich. Several examples of 

 this species which is now quite extinct in this country though 

 still living on the Continent, in S.W. France, N.E. France and 

 Belgium. Though known from several deposits in this country 

 of Pleistocene age it is not known from any newer beds. 



Bythinia tentaculata, Linn. Abundant. 



Bythinia leachii, Shepp. Not found by us, though 

 examples are in the Natural History Museum from Grays. 



Valvata piscinalis, Miill. The most abundant species. 

 Numerous examples of the form known as var. antiqua, Sow, 

 were found as well as every intermediate state between it and 

 var. depvessa, Pfr. Though the former variety is on the Conti- 

 nent considered a good species un,der the name of V . contorta, 

 Menke, in our opinion it can only be considered an extreme 

 variety of the polymorphic species V. piscinalis. We have 

 never seen recent British examples of var. antiqua though 

 according to Jordan' it still lives in Southern Scandinavia, 

 Jutland, Holland, Germany and N. France. 



Valvata cristata, Miill. Seven examples. 



Die Binnemotlusken der Nonilich gemas'iii^ten Lander von Enropa tind asien und der 

 arktischen Lander. 



