Distinguished from the Chama by the want of teeth in the hinge, and
being of a pearly substance. It is a rare shell, only found in deep
water, where it is attached to the rocks by the lower valve. Its
irregular form is perhaps occasioned by the lower valve adapting itself
to the form of the body to which it is affixed. Two species are
fluviatile and two marine.
Shell adhering, irregular, thick, pearly, inequivalve, inequilateral;
summits thick, little evident; hinge toothless, callous, thick,
irregular; longitudinal, sub-dorsal ligament, partly exterior and partly
penetrating into the shell; two oblong muscular impressions, one
inferior and anterior, the other superior and sub-posterior; with or
without an oblong incrusted callosity on the base of the valve.
Etheria elliptica.
E. semilunata.
E. trigonula.
E. transversa.
_E. elliptica._ The oval Etheria. Pl. 11, fig. 1.
Species with an oblong callosity on the anterior part of the shell;
oval, depressed, dilated towards the umbones; summits distant.
_E. semilunata._ The semilunar Etheria.
Species without callosity at the base.
FAMILY XV.
TRIDACNITES. Two genera.