The _Scuta_ generally have their lines of growth moderately prominent; occasionally they are longitudinally striated, with the lines of growth flexuous and upturned at intervals into small, sharp, hood-liked projections, which are symmetrically arranged in straight lines radiating from the apex of the valve; I have seen this structure in some specimens of _var. crispatus_ and in _var. occator_ (Pl. 2, fig. 1 _b_); and I have noticed an intermediate state in _var. communis_. The degree to which the basi-tergal corner of the valve is rounded off varies much even in _var. communis_ (Pl. 2, fig. 1 _a_, 1 _d_, 1 _e_). The articular ridge (1 _c_, 1 _e_) is broad and much reflexed; and often, but not always, distinctly hooked (1 _f_). The adductor ridge is confluent with the articular ridge, and runs straight down the valve, bounding the cavity for the depressor muscle; generally the adductor ridge is blunt, and so little prominent as barely to deserve notice; but I have seen it sharp and prominent in one specimen of _var. communis_, and it is generally prominent in _var. coccopoma_ (1 _f_), and most remarkably so in _var. concinnus_ (1 _g_). The cavity for the lateral depressor muscle is generally very slight; but in the two vars. just mentioned, and sometimes in _var. communis_, it is deep. In _var. concinnus_ (1 _g_) there is a remarkable plate developed for the attachment of the rostral depressor muscle. The scuta are coloured either dull-purple or reddish, or striped longitudinally white and blue. The surface is sometimes externally depressed in the line of the adductor ridge; and in young specimens there is sometimes, along this line, a chain of pits (1 _h_), as in full-grown specimens of _B. trigonus_ and _lævis_.