184 THE ESSEX NATURALIST.
Much of the misconception regarding our British oysters
arises probably from the want of sufficient material obtained
from different sources for comparison. Thanks to the various
friends who have responded to my appeal, I have now obtained
a large series of shells from about fifty localities, ranging from
the Shetlands to the English Channel, and from Galway to the
North Sea, with others from Norway, Sweden, and Denmark
(recent and fossil), and the western Mediterranean.
Towards the close of the Pliocene epoch of our geological
history, the genus Ostrea gradually died out in the British area.
Only a single valve of O. edulis (?) seems to have been recorded
from the Icenian Crag of Thorpe, near Norwich, by Dr. S. P.
Woodward (White's Norfolk), but the late Mr. Clement
Reid discovered a number of shells, frequently double, embedded
in a mass of sand 1/4 mile west of Sheringham, near Cromer.
These are very friable and in too bad a condition to determine,
but so far as can be made out they belong to one of the numerous
varieties assigned to O. sonora Defrance, of which M. Bagot
(Soc. Linn. Norm. 1903, p. 152), says M. de Gerville "en a repandu
nombreux echantillons sous les nom."
The fall of temperature, as the period of the major glaciation
drew on, appears to have been the principal agent in the extinc-
tion of this genus, as although it can endure considerable rigour
in congenial surroundings, it will not stand extreme cold, sudden
frost, or the influx of too much fresh land-water, nor will the
oysters spawn or the spat survive in very cold weather. This
was seen at Holy Island, near Berwick, where, according to Dr.
Johnston, the original molluscs were killed off by severe frost,
and had to be replaced by others from Prestonpans, at that time
a very prolific breeding ground.
Oysters are not very common as a rule in Pleistocene de-
posits. Messrs. Crossbey and Robertson say they could not
find the oysters at Dalmuir, or in any of the Older Glacial
Clays of the Clyde district. I have one of the older type from
Colintraive, in Bute. For this scarcity the burrowing worms
and sponges are perhaps responsible, as many of my examples
from old raised sea-bottoms are dead shells, which have been
practically eaten away by them. Where these destructive agents
are few or absent, as at March, and in the Estuarine Clays, the