THRACIA.
33
nullipore bottom at various deptlis ; but a British species
of Tfiracia {T, distorta) prefers a more secluded habitat^
and occupies the deserted holes made by Saxicava rugosa
iu limestone, or other rock- cavities, as well as tufts of
Corallina officinalis.
Genus THRA'CIA^ Leach. PI. II. f. 2.
Body oval : tubes separate.
Shell oval, nearly equilateral, rather thin, having a tubercular or shagreen -like surface : colour sometimes tinged with
yellow.
Montagu proposed his genus Ligula chiefly to receive
the species which we now assign to Thracia ; but, for
the reasons which I have given in the second volume of
this work (p. 433), it is inexpedient to retain that name
in the Mollusca.
According to Dr. Carpenter the minute elevations or
points, that roughen the surface of the shell, represent
numerous isolated cells filled with calcareous matter,
and forming a superficial coating superposed upon the
ordinary external layer ; the epidermis is extended over
these points, and sinks down into their interspaces, just
as the human epidermis covers the papillary surface of
the true skin. The proper external layer is composed of
polygonal cells, with sharply defined boundaries, having
large nuclear spots strongly resembling some of those
which are exliibited in Mya arenaria. The total quantity of animal matter or membrane contained in the
substance of the shell is extremely small, although the
cellular structure in all probability results from the calcification of animal tissue. The structure of the internal
layer is scarcely distinguishable.
The power of tension
* A> Sea-Nymph.
c5
33
nullipore bottom at various deptlis ; but a British species
of Tfiracia {T, distorta) prefers a more secluded habitat^
and occupies the deserted holes made by Saxicava rugosa
iu limestone, or other rock- cavities, as well as tufts of
Corallina officinalis.
Genus THRA'CIA^ Leach. PI. II. f. 2.
Body oval : tubes separate.
Shell oval, nearly equilateral, rather thin, having a tubercular or shagreen -like surface : colour sometimes tinged with
yellow.
Montagu proposed his genus Ligula chiefly to receive
the species which we now assign to Thracia ; but, for
the reasons which I have given in the second volume of
this work (p. 433), it is inexpedient to retain that name
in the Mollusca.
According to Dr. Carpenter the minute elevations or
points, that roughen the surface of the shell, represent
numerous isolated cells filled with calcareous matter,
and forming a superficial coating superposed upon the
ordinary external layer ; the epidermis is extended over
these points, and sinks down into their interspaces, just
as the human epidermis covers the papillary surface of
the true skin. The proper external layer is composed of
polygonal cells, with sharply defined boundaries, having
large nuclear spots strongly resembling some of those
which are exliibited in Mya arenaria. The total quantity of animal matter or membrane contained in the
substance of the shell is extremely small, although the
cellular structure in all probability results from the calcification of animal tissue. The structure of the internal
layer is scarcely distinguishable.
The power of tension
* A> Sea-Nymph.
c5