Pvn H,i I'UitlS

MYTILUS.
105
end to an obtusely rounded edge, gibbous with a gradual slope
towards the front and sides, rather thin and glossy : sculpture,
concentric and indistinct lines of increase: colour bluish or
purple : epidermis dark brown or olive, occasionally yellowish
or having a reddish tint, rarely black, microscopically striated
lengthwise and shagreened, sometimes puckered in particular
on the ventral
spots : margins rather thick, nearly straight
rounded in front : beaks
side and arched behind, obliquely
blunt, more or less divergent, placed a little below the termination
or point of the shell, which is formed by the dorsal
edge of the first-formed part or nucleus beneath the beaks
;
is an imperfect lunule or heart-shaped impression, which
often appears to be repeated, showing the marks of successive
growth: hinge-line nearly straight, occupying almost the
upper half of the dorsal margin: ligament very thick and
strong :
hinge-plate partly folded over the ligament, the middle
furnished with 3-10
of which is : consequently exposed hinge
small tubercular teeth in each valve : inside chalky-white,
slightly nacreous at the edges, and pitted (as in Unio margaritifer)
by the impressions of the mantle : muscular scars very
distinct. L. 2-3. B. 1-2.
Yar. 1. ungulata. Shell larger, more pointed at the narrower
end, and sometimes angular at the other, with the
ventral margin incurved : beaks more widely separated than
usual. M. ungidatus, Linn. Syst. Nat. p. 1157.
Yar. 2. incurvata. Shell stunted and bent, often marked
by longitudinal ravs of purple. M. incurvatus, Pennant, Brit.
Zool. iv. p. Ill, pi. 64. f. 74.
Yar. 3. Galloprovincialis. Shell broader and natter, with
the ventral margin somewhat prominent and the posterior
angle more acute than in the typical form : beaks incurved.
M. Galloprovincialis, Lamarck, An. sans Yert. vi. p. 126.
Yar. 4. pellucida. Shell thin, narrower, less gibbous, and
beautifully streaked by longitudinal rays of dark brown or
purple, which are
irregularly disposed. M. pellacidus,
Penn.
63. f. 75.
op. c. p. 112, pi.
Habitat :
Abundant
everywhere and gregarious, from
high-water mark to the depth of a few fathoms, on a
rocky, stony, or muddy
bottom. Var. 1. Cornwall and
Channel Isles, at low spring tides. This variety is the
f5