BRITISH OYSTERS : OLD AND NEW.
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appressed. My largest example measures, length 130 mm.,
breadth 140 mm. ; my smallest (plate xvi., fig. 18), 3 by 23/4
inches, the lower valve expanding at the shoulders to 31/2 inches.
Many of these Western forms attain a great size, height 51/2 to
6 inches, breadth 41/2 inches, and are round and solid ; the ribs or
folds on the lower valve being narrow to moderately broad,
so that, when the concentric laminae are raised, as they usually
are, the interrupted ribs become vaulted or fimbriated, especially
so in Jersey shells. The corneous plates in the upper valve
are broadly laminate, and extend considerably beyond the
margins of the valves.
Da Costa's notice of the growth of the lamellar horny plates
is very clear and may be quoted here (British Conchology, 1778),
as it so exactly describes the ornament and growth of the Western
oyster. "Usually about 3 inches long, and less in breadth, the
shells vary in size and shape from their adhesion to other bodies
in different places.
" The upper or flat valve is of a dirty brownish hue, roughly
plated or made up of transverse flakes exceedingly thin. These
lie close, compact and strongly set together on the upper part,
well towards the middle of the shell, from thence to the bottom
they are more loosely set, become finer and thinner, and more
extended beyond one another. As they approach the lower
margin they are generally so loose, separate and extended,
as to foliate the shell very finely, even much beyond the edges
of it.
"The under valve is very rugged, whitish or generally greenish;
the leafed structure seldom shows such fine foliations as the
upper valve ; these are chiefly apparent on the edges of the
wrinkles that cross the shell, which they furbelow or plait. These
are commonly of a purplish colour; the valves wrought with
several irregular, prominent, longitudinal ribs."
Forbes and Hanley (Brit. Moll. ii., p. 307, pl. liv.) notice the
"dissimilarity of aspect between the sleek looking valves of the
flattened native oyster of our markets, and the more coarse (!)
and rugged, but far more beautifully sculptured and coloured,
solitary individuals which are ordinarily termed rock oysters."
The shell figured by these writers is very different from that of
Jeffreys (op. cit., fig. 1), and represents a shell not very common
in the cabinets of collectors. Their description reads like an