THRACIA.
35
on every part except the posterior side, which is covered with
numerous concentric rows of tubercles interspersed with fine
striae that appear to radiate from the tubercles : colour milkwhite : epidermis membranous, creamcolour : margins semicircular on the anterior side, moderately curved in front, with
a shght indentation or flexuosity towards the posterior side,
which is more rounded than truncated ; posterior dorsal margin
sloi)ing* and straight ; anterior dorsal margin slightly curved :
beaks projecting, with an abrupt excavation underneath, caused
by the compression or fracture of the hinge ; this part is defined by a sharp but irregular ridge in each valve : ligament
exceedingly small (being only visible in fresh specimens),
placed close to the hinge on the posterior side; it is dark
horncolour : cartilage golden-yellow, contained in a triangular
and shallow cup, which is sohd, attached to the hinge-plate
by a ledge, and projects inwards horizontally and at a right
angle with the hinge ; from the lower part of this cup or cartilage-pit in each valve runs an obliciue and sharp ridge to the
posterior adductor muscle, and the shell is considerably thickened in that part : hinge-line obtusely angular : hinge-plate
narrow and thin : ossicle falciform, clasping the hinge close to
the beak on the anterior side : inside chalky- white, except the
muscular scars and below the cartilage-pit, where the surface
is pohshed and nacreous ; it is furnished with a shght rib in
the line of fracture ; edges sharp : pallial and muscular scars
nearly marginal. L. 0*85. B. 1-3.
Yar. curta.
Shell more oval, or longer relatively to its
breadth.
Habitat : Laiid^s End to Unst, from 4 to 60 f. ; and
at low water, spring tides, on the coasts of Kerry and
Galway. The variety is from Shetland and Cork Harbour.
Fossil in the Coralline Crag (S. Wood) ; Christiania (Sars); Palermo (Philippi). The extra-Britisli
distribution comprises Iceland, the Faroe Isles, Scandinavia (3-30 f.), the north of France, Adriatic, Naples,
and Sicily.
The course of striation, or the arrangement of the
microscopical granules, in this shell is the reverse of that
in Lyonsia Norvegicaj viz. transverse instead of longi-
35
on every part except the posterior side, which is covered with
numerous concentric rows of tubercles interspersed with fine
striae that appear to radiate from the tubercles : colour milkwhite : epidermis membranous, creamcolour : margins semicircular on the anterior side, moderately curved in front, with
a shght indentation or flexuosity towards the posterior side,
which is more rounded than truncated ; posterior dorsal margin
sloi)ing* and straight ; anterior dorsal margin slightly curved :
beaks projecting, with an abrupt excavation underneath, caused
by the compression or fracture of the hinge ; this part is defined by a sharp but irregular ridge in each valve : ligament
exceedingly small (being only visible in fresh specimens),
placed close to the hinge on the posterior side; it is dark
horncolour : cartilage golden-yellow, contained in a triangular
and shallow cup, which is sohd, attached to the hinge-plate
by a ledge, and projects inwards horizontally and at a right
angle with the hinge ; from the lower part of this cup or cartilage-pit in each valve runs an obliciue and sharp ridge to the
posterior adductor muscle, and the shell is considerably thickened in that part : hinge-line obtusely angular : hinge-plate
narrow and thin : ossicle falciform, clasping the hinge close to
the beak on the anterior side : inside chalky- white, except the
muscular scars and below the cartilage-pit, where the surface
is pohshed and nacreous ; it is furnished with a shght rib in
the line of fracture ; edges sharp : pallial and muscular scars
nearly marginal. L. 0*85. B. 1-3.
Yar. curta.
Shell more oval, or longer relatively to its
breadth.
Habitat : Laiid^s End to Unst, from 4 to 60 f. ; and
at low water, spring tides, on the coasts of Kerry and
Galway. The variety is from Shetland and Cork Harbour.
Fossil in the Coralline Crag (S. Wood) ; Christiania (Sars); Palermo (Philippi). The extra-Britisli
distribution comprises Iceland, the Faroe Isles, Scandinavia (3-30 f.), the north of France, Adriatic, Naples,
and Sicily.
The course of striation, or the arrangement of the
microscopical granules, in this shell is the reverse of that
in Lyonsia Norvegicaj viz. transverse instead of longi-