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.