N. margaritacea.
N. rostrata. The beaked Nucula.
Species of which the margin is entire.
N. margaritacea. The pearly Nucula. Pl. [11], fig. 7.
Species of which the margin is crenated; numerous regular pectinated teeth; obliquely ovate, trigonal; striæ minute and almost obsolete; covered with a greenish epidermis; inside silvery, pearl-like.
FAMILY XII.
Trigonacea. Two genera.
1. Trigonia. One species.
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.