So called from the appearance caused by three holes or cavities on the surface of the lower valve.
Shell irregular, orbicular, inequivalve; the inferior valve almost flat, and marked on the interior with four muscular impressions, sometimes very deep, and of which the two sub-central are sufficiently connected to form but one; the superior valve like a Patella, more or less convex, with four very distinct muscular impressions, rather distant.
C. personata. The masked Crania. Pl. [16], fig. 4.
Orbicular; upper valve gibbous and conical, lower valve flat, with three perforations.
FAMILY XX.
Brachiopoda. Three genera.
1. Orbicula. One species.
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.