Pvn H,i I'UitlS

376 TELLINID^E.
earlier stages of growth and abruptly becoming
white after-
wards : epidermis minutely wrinkled, fibrous at the sides and
edges of the shell, light brown, or occasionally greenish, mostly
effaced in : full-grown specimens margins rounded in front and
on the anterior side, somewhat produced on the posterior side
and ending in an obtuse angle, with a greater or less degree
of flexure, and frequently with an angular ridge running from
the posterior end to the beaks ; dorsal margin sloping with a
slight curve : beaks small, inflected, nearly contiguous ; urnbones
scarcely projecting : lunule lanceolate, not very distinct :
ligament strong, prominent, dark horncolour, now and then ex-
tending between the beaks to the lunule : hinge-line curved :
hinge-plalet hick and broad, occupying between a third and a
fourth of the circumference : teeth fixed in a shallow cavity or
depression in the middle of the hinge-plate, and set either
consist of two small
across it or in opposite directions ; they
symmetrical cardinals in each valve, the posterior in the right
valve and anterior in the left being bifid or cloven lengthwise
: inside glossy and nacreous, often partaking of the ex-
ternal colour, and microscopically wrinkled : pallial scar large,
triangular, extending in a line parallel with the front margin :
muscular scars deep and distinct. L. 0*7. B. 0*8.
Yar. 1. attenuata. Shell smaller, more compressed, and of a
thinner consistency, often eroded ; epidermis also thin, partially
iridescent ; teeth very slight ; inside chalky-white.
Var. 2. minor. Shell smaller and more triangular. L. 5.
B. 0-55.
Var. 3. nivea. Shell of the same size as the last variety,
but more compressed, snow-white.
Habitat :
and mud between tide-marks
Sand, gravel,
on every shore. Var. 1. Southampton (J. G. J.); Or-
well estuary (Dr. Clarke). This is the Baltic form.
Var. 2. Bantry Bay (Humphreys and J. G. J.) Var. 3.
Llanrhidian Marsh in the estuary of the Burry River,
Glamorganshire (J. G. J.) Our usual form (which may
be termed soliduld) abounds in all the later tertiary deposits,
including the '
boulder-clay or tilP and the Mammalian
Crag. It may therefore be regarded in the main
as a northern species ; but it is likewise common in