200 THE ESSEX NATURALIST.
85 mm. The valves in the Neolithic clays of the Forth Estuary,
although much decayed, present the same type. The pointed
anterior margin seems to be a typical feature in many of the
Scottish Ostreidae, as it occurs in the shells of the Neolithic age
at Dunagoil, W. Scotland, and also in those living at Scalpa
and Loch Sween, Argyll. It is also found present in the Helston
(Falmouth) shells, and in the eastern beds at Kelsey Hill,
March, and Felixstowe (Roman) (plate xiii., fig. 5). It may be,
as already suggested, a sign of full growth, as seen in the O. tincta,
O. tarentina, and the Fairlie shell. These latter have all
immersed top valves, and in this respect vary from the
Forth examples, which are equal margined ; and O. rutupina
seems to be an oblong shell up to a certain point, and then to
enlarge laterally.
VAR. CELTICA var novo.
This variable group includes the larger portion of the oysters
having their original home in the northern seas ; Shetland,
from the abundance of its shells, both dead and living, appearing
to be its metropolis. Their chief traits are their size and strength,
and the strong costae on the lower shell, varying from ribs close-
set (plate xiii., fig. 8), to others broader and wider apart. This
group ranges from the Shetlands to the Irish Channel, and
round to Cork, but as before stated shells are now seldom ob-
tained living except by trawling, and as dead shells in the Kitchen
Middens on the Scottish Shores, and the scalps lying between
tide marks in many places. It does not appear to come into
the English Channel, or very far down into the North Sea. The
shells are nearly always thick, strong, and massive, and range
in shape from pyriform, especially in the more northern localities,
to rounded or ovate further south, though no fixed rule can be
laid down. Broadly stated, the elongated shells seem to be
the oldest type.
I owe to the good offices of Mr. Duthie, of Lerwick, a series of
oysters from Shetland, once apparently rich in these molluscs,
but now found only at two localities, Burra and Basta Voe which
yield a few examples, banks of dead and old shells now repre-
senting the old oyster fauna.
The shells are fairly strong and stoutly built, and of a good
size, my largest specimen being 5 inches (125 mm.) in length, the