ALLUVIUM AT WALTHAMSTOW, ESSEX. I9 



from the Pleistocene of Barnwell and the Holocene of Exedown 

 (near Wrotham), Crossness and the Plampshire tufaceous 

 deposits. 



Clausilia bidentata, Strom. This widely distributed form 

 has hitherto been undetected in these beds. It was represented 

 by six examples. 



Succinea elegans, Risso, proved a more abundant form than 

 S. putris, Linn. One example of ths former, the largest we have 

 seen, measured no less than 21 mm. in length. The largest 

 individual oi S. pittvis was i8'5 mm. in length, the average size 

 being 12-5 mm., whilst the average of 5. elegans is rather less. 



Carychium minimum, Miill., was not common, but it is 

 a new record. 



Limnaea auricularia, Linn, on the other hand was very 

 abundant, many of the examples attaining a large size, but the 

 finest of all were obtained from the modern deposits. 



Limnsea pereger, Miill., was equally common with the 

 last-named in the older beds, but was decidedly scarce in the 

 more recent deposits. 



Limnaea stagnalis, Linn., attained a large size, and was 

 not uncommon. The largest individual measured 52 mm. in 

 height, with a maximum breadth of 22 mm. 



Limnsea truncatula, Miill., was another common form, 

 and was decidedly above the average in size. 



With the exception of Planovhis glabev, Jeff., and P. lineatns, 

 Walker, all the British species of Planorbis were present. It 

 was the material from these beds that first enabled us to 

 differentiate the form P. stroemii. West, which had not hitherto 

 been noted in these islands, either living or fossil.^ 



The species is near to P. alhiis Miill, but is distinguished 

 from it by its larger size, the constant presence of a keel, and 

 the absence of spiral striae : it also resembles an almost keelless 

 variety of P. cavinatus. 



At Walthamstow P. stroemii was a far more abundant form 

 than P. alhus in the older beds, but in the modern deposits the 

 former is very scarce, whilst the latter is abundant. In our 

 opinion the examples in the recent beds have been derived from 

 older deposits. Mr. A. C. Johansen, of the Zoological Museum, 



2 Proc Malacological Soc. Lond. Vol. IV. (1901), p. 236. 



