1. Genus Trigonia. Pl. [VIII].
Animal. Entirely unknown.
Shell. Subtrigonal or suborbicular, thick, regular, equivalve, inequilateral; summits but slightly prominent, little flexed, anterodorsal; hinge complex, dorsal, dissimilar; two thick oblong teeth joined angularly under the summit, strongly furrowed upon the right valve, penetrating into two excavations of the same form, also furrowed, in the left valve; ligament postapicial; two distinct muscular impressions, not united by a band. Inhabits the Australian seas. One living species. Fifteen fossil.
- Trigonia pectinata.
2. Genus Castalia. Pl. [VIII].
Animal. Body large, slightly compressed, or moderately thick, more or less oval; mantle with thick edges, simple or broken, open in all its circumference except towards the back a kind of small, incomplete tube, furnished with two rows of somewhat elongated cirri for the respiratory cavity; flamelliform and trenchant.
Shell. Subtrigonal, equivalve, inequilateral; umbones eroded, covered with epidermis, and flexed anteriorly; hinge with two lamellar teeth transversely striated, one distant, posterior and shortened, the other anterior, long, and lateral; ligament exterior. Habitation unknown. One species.
- Castalia ambigua.