BRITISH OYSTERS: OLD AND NEW. 191
east, show incipient costal ridges which I have not noticed in
western shells. The "plaits" or shoots shown on the under
valve increase yearly, and are fairly regular up to five years
of age, when they have a tendency to become irregular, and often
foliaceous at the sides, increasing in breadth from time to time,
thickening internally. The shell is often camerated, or becomes
spongy and loose in texture, and subject to the attack of many
lithophagous annelids, and boring sponges.
In shape the shell is variable, mostly rounded, or if the apex
is produced, pyriform. The round form frequently becomes
falcate, the lower portion curving, usually to the left. This is
very noticeable in the O. estuarii group. The curvature and
position of the apex, whether turned to the left (its normal place)
or to the right, does not seem material to the growth of the
mollusc, its position being determined by the way the spat falls.
The umbo of the lower valve is frequently elongated in old shells,
and is mostly chambered.
The ligamental area in the type O. edulis and its allied forms
is trigonal, enlarging with age and exhibiting lines of accretion
in normal conditions, but is subject to the mode of growth by
the umbo, sometimes becoming long and attenuated.
The disposition of the ligamental area and the apex has little
relation to the rest of the shell, except so far as the body position
of the animal is concerned, the position of the adductor scar
agreeing with the right or left inclination of the apical portion
of the shell. The thickness of the aged or enlarged apex is
illusory, as in one Selsea Bill shell, with area 25 mm. long, and
12 mm. thick, it is made up of hollow concamered chambers.
Having attained maturity, the animal ceases to grow bodily,
but enlarges the shell by the deposition of additional shelly matter,
seeming as if it shrank itself in the process of doing so. The
so-called bottom valve in deep water individuals is not infre-
quently coated with masses of Serpulas and Balani, parasites
seldom seen on the flat valves.
OSTREA EDULIS Linne
While all writers agree that Linne uses the name ostrea edulis,
some of them differ as to which of its numerous varieties he
referred to. His diagnosis (Syst. Nat., x., p. 699, 1758) "testa