242 A SHELL-BEARING GRAVEL AT DUMBALL ISLAND 
The surface of the island at this point is about 21 feet 
ubove Ordnance Datum, the section therefore passing 
down to 45 feet below Ordnance Datum. Below this 
depth the Marl is reached, beyond which the excavations 
■are not carried. 
In order to determine what shells were present in the 
deposit, a small boxful of the material was obtained, 
and carefully washed and sifted. An examination of the 
washed material resulted in the discovery of no less than 
twenty-four species of mollusca, Tellina halthica pre- 
dominating. 
The identification of the collection proved extremely 
interesting, as it showed the deposit contained a com- 
mingling of land, freshwater, and marine forms, together 
with a portion of a claw of a crab. 
The following is a list of the shells obtained : — 
Land and Freshwater — 
Vitrea (Polita) cellaria. 
Hygromia (Fruticicola) hispida. 
Pyramidula (Gonyodiscus) rotundata. 
Helix hortensis. 
Cochlicopa lubrica. 
Succinea putris. 
Planorbis albus. 
,, glaber (parvus), 
,, complanatus (umbilicatus). 
Limnea peregra. 
Ancylus fluviatilis. 
Cyclostoma elegans. 
Bithynia tentaculata. 
,, leachii. 
Valvata piscinalis. 
Pisidium pusillum. 
Paludestrina stagnalis. 
Marine — 
Littorina obtusata. 
,, rudis 
Neritina fluviatilis. 
Leuconia bidentata. 
Vallonia pulchella. 
Scrobicularia piperata. 
Tellina, balthica. 
