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 0 . 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 | 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 0 . sonnra 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 
