Pvn H,i I'UitlS

PECTEN. 51
A. Upper valve more or less convex : hinge-line
1. Pecten pu'sio"*, Linne.
ribbed across.
Ostrea pusio, Linn. Syst. Nat. p. 1146. T.^usio, F. & H. ii. p. 278, pi. L.
f. 4, 5, and li. f. 7.
Body vermilion or yellowish -white with a brown tint, or
particoloured : cirri numerous, short, and blunt, arranged in
from 5 to 7 rows : ocelli large and few in number.
Shell varying in shape according to age, being when young
considerably longer than broad, and regular, but in its adult
state broader in proportion, and distorted or twisted in consequence
of its fitting the cavities and sinuosities of the bodies
it is almost
to which it is fixed ; in the earlier stage of growth
equivalve, but afterwards the upper valve becomes usually the
larger and more convex of the two ; sides nearly equal ;
rather solid, and not : glossy sculpture,
it is
about 70 narrow and
sharp ribs which are alternately large and small, crossed by
numerous transverse plates, which by their intersection form
scales or prickles on the crests of the ribs ; the whole surface
is exquisitely marked by microscopical longitudinal striae which
diverge from each successive layer of growth ;
in the fry these
stria3 only are visible, the ribs not then existing : colour red-
dish, yellowish, brown, or white, or of intermediate shades,
variegated by straight or diverging streaks or blotches of some
of those tints : margins rounded in front and at both sides, and
notched or indented by the impression of the ribs ; in the
young the upper edge of the angle on the right-hand side,
which lies under the large ear, has a row of curved spines,
like the teeth of a saw beaks :
prominent
which are arranged :
ears of unequal size, especially in the young, that on the lefthand
side of .the upper valve and -on the right of the lower
valve being the largest ; all of them are sculptured like the
rest of the shell, the ribs diverging from each side of the beak
outwards ; the right-hand ear of the lower valve is notched at
the base, and it is smaller than the opposite one on the left
hand of the upper valve, in order to make an opening and
short but
passage for the : byssus hinge-line straight : cartilage
strong : ligament long and slender : hinge-plate strengthened
by a thick and knob-like rib on each side of the beak, to form
the sides of the cartilage-pit: inside pearly, microscopically
* A youngster.
D 2