BRITISH OYSTERS : PAST AND PRESENT. 301
they are quite unlike a series, sent me by Mr. Calderwood, obtained
at a depth of 8—12 feet near the Bridge of Allan beyond Stirling,
almost half way across Scotland. These are long, in proportion
to breadth, but are of the true estuary type, not very large, 21/2
by 11/2 inches on an average.
Jura Oysters attained a fairly large size (one old dead valve
I possess reaches to 5 by 4 inches diameter), and are often very
fresh looking ; one specimen having a large double Anomia affixed
to it. They are, like a large proportion of these western oysters,
of the same class as those of the Estuarine Clays of the Nar Valley
and the N.E. of Ireland. The deep sea Dogger and Welsh shell,
var. Tenbiensis, is probably a surviving representative of this
old group.
Some well marked sculptural features survived for a long
period ; thus the Selsey type (Plate XV, fig. 16 ante) has its exact
replica in a Nar Valley shell in the Norwich Museum. I have
it from Jura in Mull. The beautifully ornamented (lower)
valves are very shallow. A Jura shell of this type has the flat
upper valve recessed into the lower one, a frequent habit in
Western shells.
Another long lived type is a deep, almost hemispherical, basin-
shaped shell with very pronounced sculpture, both in the Nar
Valley and Loch Don in Mull, where the presence of the shell
was unknown until Mr. Calderwood discovered their former
existence associated with many pointed branches of trees,
perhaps an early fascine to encourage their growth.
Natural agencies, including current action, have had much
to do with the in-bringing of Atlantic and Southern forms to our
Western coasts such as the O. scaeva to which I have already
referred, and which has been sent me by M. Dautzenberg from
St. Malo, O. cochlear, O. atlantica, and the beautifully coloured
and delicately ornamented O. Montagui and O. Devonensis.
Turton's O. parasitica on floating timber (p. 194 ante), may be
another example.
At what particular period of our geological history the pointed
Rutupinian type made its appearance is not very apparent.
Its distribution in one or other form, for the type is variable,
over such a wide area from the Thames mouth to S. and W.
Scotland, presumes a great antiquity. The English type goes
back as far as the March gravels. It is closely related to the