Supposed to be in very deep places in the sea; it is a strong,
beautiful, pearly shell, sub-trigonal or sub-orbicular; thick, regular,
equivalve, inequilateral; summits slightly prominent, recurved,
anterodorsal; hinge complex, dorsal, dissimilar; two large oblong teeth
laterally compressed, joined angularly under the summit, strongly
furrowed on the right valve, penetrating into two excavations of the
same form, equally furrowed on the left valve; ligament exterior and
marginal; two distinct muscular impressions.
_T. pectinata._ The pectinated Trigonia. Pl. 11, fig. 4.
Species sub-orbicular, with radiated or divergent, prominent, and
somewhat scaly ribs; inside pearly; margin crenellated.