Pvn H,i I'UitlS

DONAX. 403
it an octagonal appearance ; these lines terminate at the orifice
in eight small fimbriated prongs, which fold over and enclose
the tube whenever anything passes into it ;
the excurrent tube
has only six of these lines and as many terminal prongs ;
when the tubes are withdrawn, they are protected by
a small
pouch formed out of the mantle ; siphonal
a very pale orange colour :
gills large, suboval, light brown ;
upper pair much smaller and shorter than the lower : palps
rather long, triangular, pale yellow, smooth externally and
orifices and cirri of
distinctly striated internally : foot very large, much compressed,
bevelled to a sharp edge and lanceolate point when fully extended
; but when at rest or half exserted, it is puckered and
transversely ridged.
Shell triangularly oblong, not very inequilateral, somewhat
compressed but more convex towards the beaks and sloping
from that part to the margins, rather thick, opaque, glossy and
polished: sculpture, numerous fine longitudinal stria?, which
radiate from the beak but do not extend to the anterior or
larger side, and only cover about two-thirds of the surface ;
they are more crowded on the posterior slope ; the posterior
side is also marked with a few concentric or transverse stria?,
which are scratch-like and veiy irregularly disposed, never
reaching to the ventral area ; these last stria? are usually
llexuous, and observable on one or more parts of the posterior
area but never on all parts of it ; they are not found in the
earlier stages of growth, and occasionally (but rarely) they are
where the two sets of stria? intersect each
altogether wanting ;
other a partial decussation is of course the result : colour
yellow, brown, or purple of different shades, often variegated
(especially in the young and half-grown specimens) with three
rays of white, one proceeding from the beak, another crossing
the ventral range, and the third running along the posterior
ridge ; the lines of growth are generally marked by broad con-
centric bands of a deeper hue ; sometimes the colour is uniform
milk-white : epidermis thin and smooth, like varnish,
delicately and microscopically
striated in a transverse direc-
tion : margins gently curved in front, with a slight indentation
or sinuosity near the posterior side, which is gibbous, strongly
angulated, obliquely but not abruptly truncate, and ends in a
blunt and somewhat rounded point ; the ridge which encloses
a great part of this side is prominent and extends from the
beak in each valve, enclosing a kind of enormous lunule with
projecting lips; anterior margin semioval and wedge-shaped;