A SHELL-BEARING GRAVEL AT DUMBALL ISLAND 245 
All the land and freshwater species in this list are found 
in the North Somerset vice-county as defined by the 
Conchological Society of Great Britain and Ireland, and 
all, with the exception of Planorhis glaher (parvus) and 
Pisidium pusillum, in the West Gloucestershire vice- 
county. 
A little consideration will show that this association of 
mollusca is of considerable interest from a geological no 
less than a zoological point of view. Of the molluscan 
fauna of the land in past geological time we know very 
little. Extensive deposits formed upon the land can 
hardly be looked for ; marine and estuarine deposits are 
common enough, yet in this case we have an estuarine 
deposit containing a large majority of land molluscan 
forms. 
The manner in which these members of the land mol- 
luscan fauna have been brought to their present position 
is easily understood. It will have been noticed that eleven 
species, or twelve, if we add Paludestrina stagnalis, are 
freshwater forms, inhabiting ponds, ditches, and streams. 
During freshets and fioods, and probably also during the 
ordinary course of drainage, the empty shells of these 
forms would easily be swept into the Avon, and carried 
onwards until the current received a check, when the 
Avon reached the Channel. The empty shells of the land 
species would not be so easily swept into ditches or streams 
to be carried onwards, hence their rarity and the absence of 
many of the commoner species of Helix, etc. The depth of 
the deposit indicates that it must have been formed at a 
time when the North Channel was not only navigable but 
deep, and probably serving as the main outlet of the 
river. 
The presence of the marine species can be accounted 
for by the vast volume of sea water, and the consequently 
strong current which set up the river on a rising tide> 
