Greatly resembling a Patella, for which it is often mistaken on account
of the lower valve being very thin, flat, and adhering.
Shell sub-orbicular, very compressed, inequilateral, very inequivalve;
the inferior valve very thin, adherent, and imperforated; the superior
valve like a Patella, with a summit more or less inclined towards the
posterior side.
_O. Norwegica._ The Norwegian Orbicula. Pl. 17, fig. 4.
Upper valve in the form of a depressed cone, with a summit produced and
pointed.