The _Tergum_ (Pl. 2, figs. 1 _i_ to 1 _o_) is broad, with a generally closed longitudinal furrow; this furrow is open in young specimens, and it is often, but not always, open in rather large specimens of _var. occator_; it is always open in _var. d'Orbignii_ (Pl. 2, fig. 1 _m_), and sometimes in _var. concinnus_. Apex barely beaked, except in _var. spinosus_, in which it is sensibly produced. Spur placed either very nearly in the middle of the basal margin, or when least medial, it stands at above its own width from the basi-scutal angle; yet in some specimens of _var. occator_ the spur is less than its own width from this angle. The basal margin (1 _i_), on opposite sides of the spur, either forms a nearly straight line, or the scutal portion descends lower than the carinal portion, and curves very regularly towards the spur; this is the case in _var. d'Orbignii_ (1 _n_), and in some specimens of _var. occator_. The carinal half of the basal margin generally forms an angle with the spur of only a little above a rectangle. The spur varies a little in length and breadth, but never exceeds one fourth of the greatest breadth of the valve. The scutal margin is broadly inflected, the inflected portion forming either a right angle, or somewhat less than a right angle, with the exterior surface of the valve. Internally the articular ridge is prominent, and is either considerably or slightly curved; it extends down either about half, or three fourths, of the length of the valve. The spur is produced for a considerable distance up the internal surface of the valve as a prominence. The crests for the tergal depressor muscle are very feeble. In one specimen, in which both the shell and operculum had undergone much disintegration, the scuta and terga were calcified together.