ALLUVIUM AT WALTHAMSTOW, ESSEX. 21 



beds, and 3 from the modern deposits. These last are : — 

 Helicella cantiana (Mont.) 

 Sphserium rivicola, Leach. 



lacustre (Miill.) 



The first-named must be considered a recent introduction, 

 because though widely distributed over Essex at the present 

 time, it has not hitherto been found fossil in any deposit in the 

 country, however modern, and the available evidence seems to 

 show that it was introduced at a later period. Sphocvmm rivicola, 

 on the other hand, is known from both the Pleistocene and 

 Holocene. Its absence from the Shell-marl is probably acci- 

 dental, and the same may be said of Sphcevium lacustre. The 

 four species which were not identified by us in these recent 

 excavations are : — 



Acanthimda lamellata (Jefi".) Vivipara contecta (Millet.) 



Helicella caperata (Mont.) Unio pictonim (Linn.) 



The last-named was to be seen in the modern beds, but not 

 in the older deposits. 



We have already noted two species, Planovhis stroemii, West, 

 and Pisidium supinum, A. Schm, as being quite extinct in this 

 county, though common in these deposits, but the remaining 

 64 species are all living in the area drained by the River Lea. 

 One thing is noteworthy, and that is the almost total absence of 

 abnormal or monstrous forms, with the exception of Planovhis 

 stroemii. This form is very often slightly distorted. There were 

 a few " mended " examples of Helix nemovalis and Helicigona 

 avhiistovum ; two slightly scalariform specimens of Bithynia 

 tentaculata and one of Limnaa palustvis ; two distorted examples 

 of Planovhis spivovhis and three of P. vovtex. These are all 

 observed, although we have had many thousands of shells 

 through our hands, 



A great advance has been made in our knowledge of these 

 beds in the last few years. In 1890 only 40 species were known 

 as occurring in them. In 1897 we were able to list 52, and now 

 the record stands at 70, or if we include the forms from the 

 modern deposits, at 73. Fresh excavations will no doubt add 

 to the list, since there are many forms living in the Lea Valley 

 which are unknown from these beds. We would urge all mem- 

 bers of the Essex Field Club to lose no opportunity of collecting 

 from any future exposure in the Lea alluvium. 



