l6 THE NON-MARINE MOLLUSCA OF THE RIVER LEA
size and in their composition. Occasionally all traces of
mollusca had vanished, whilst in others only a few still
remained; but as a rule they were very abundant and well
preserved. Many were coated with calcareous matter, com-
pletely covering the shell. This condition is also met with in
some of the Irish lakes where the dead shells are drifted ashore.
It is also found in the Holocene deposits of the Thames, and in
the Pleistocence High Terrace Gravel at Swanscomb, Kent.
It has been suggested that the primary cause of this incrustation
may be the coating of algoid growth that invested the
test during the life of the animal. As a rule, more aquatic forms
were present in these marls than terrestial, although occasionally
the reverse condition prevailed. We obtained 66 species, of
which 18 have not hitherto been recorded from these beds, whilst
four, before observed, were not found by us :—
LIST OF SHELLS FROM SHELL-MARL.
Agriolimax agrestis (Linn.)
Vitrea crystallina (Mull.)
„ alliaria (Miller)
„ cellaria (Mull.)
,, nitidula (Drap.)
,, radiatula (Alder)
„ nitida (Mull.)
,, fulva (Mull.)
Arion ater (Linn.)
Punctum pygmaeum (Drap.)
Pyramidula rotundata (Mull.)
Hygromia granulata (Alder)
„ hispida (Linn.)
Vallonia pulchella (Mull.)
Helicigona lapicida (Linn.)
,, arbustorum (Linn.)
Helix aspersa, Mull.
,, nemoralis, Linn.
,, hortensis, Mull.
Cochlicopa lubrica (Mull.)
Pupa muscorum (Linn.)
Vertigo antivertigo (Drap.)
Vertigo pygmaea (Drap.)
Balea perversa (Linn.)
Clausilia laminata (Mont)
,, bidentata (Strom)
„ putris (Linn.)
Succinea elegans, Risso.
Carychium minimum, Mull.
Ancylus fluviatilis (Mull.)
Velletia lacustris (Mull.)
Limnea auricularia (Linn.)
pereger (Mull.)
„ palustris (Mull.)
,, truncatula (Mull.)
,, stagnalis (Linn.)
Planorbis corneus (Linn.)
„ albus, Mull.
„ stroemii, West.
,, nautileus (Linn.)
,, carinatus, Mull.
„ marginatus, Drap.
,, vertex (Linn.)
,, spirorbis, Mull.