204 THE ESSEX NATURALIST.
The upper valve exhibits numerous close-pressed fairly broad
lamellar growths, the edges hardly rising above the flat surface,
the umbonal area is usually depressed or flattened. Inside, the
inhabited portion of the shell in the older examples is pyriform,
becoming attenuated towards the wide and generally shallow
ligamental area, terminating in an acute and sinuous point in
the younger shell, and widening below in a posterior lateral
direction.
The variation in shape is due to the way in which the accre-
tionary shell-matter has been disposed on the anterior side of
the umbonal region (plate xiv., fig. 12) and this may extend to as
much as 11/2 inch beyond the natural margin of the inhabited
portion ; the breadth over all may be from 31/2 to 5 inches, the
narrowest shells being usually the longest. Inside the variation
is much less apparent. Plate xiv., fig. 12, gives a fair idea
of the habitable shell.
A. S. Stewart was the first to call attention to these shells.
In a paper published many years after its reading (Proc. Belfast
Naturalists' Field Club, vol. xx., p. 17) he mentions finding
immense specimens of the solitary deep water variety of the
oyster O. hippopus. Canon Grainger (Dublin University Geol. and
Bot. Proceedings, vol. 1, 1859), says the shells occur of immense
size in innumerable myriads. The earliest figure I have seen of
this type of oyster is in Brown's Rec. Con. Gt. Britain (pl. xxii., fig.
19), being that of an old Firth of Forth specimen. A thick bed
of these shells occurs at Grangemouth, where the shells are large
and solid to a degree seldom attained by the normal form in the
present area. They also occur near Micklewood, five miles west
of Stirling, under 18 feet of clay, but several feet above high
water mark. Nordmann figures a shell of this kind from the
Danish Middens.
In the Nar Valley, Norfolk, at West Bilney and Narford
"large antiquated specimens are common" (S. Woodward,
Geol. of Norfolk). A specimen in the Museum of Practical Geology,
London, measures 7 by 4 ins. in its dimensions, length of ligamental
pit 2 ins., breadth 21/4ins. Marshall and Praeger correlate the
Irish shell with O. hippopus, but it does not agree with the
North Sea shells, nor does it occur in the English Channel
It may be noticed that all these deep estuary oysters are very
dark in colour, almost black.