_Scutum._--This valve is almost triangular, with the basi-tergal corner more or less rounded off. The prominent lines of growth are sometimes crossed by longitudinal striæ. Internally, the articular ridge projects to a very different degree in the different species; its lower end is sometimes (as in _B. lævis_, Pl. 4, fig. 2 _c_) produced downwards as a small, sharp, free style; there is always an articular furrow receiving the inflected margin of the tergum. There is always an impression left by the attachment of the adductor scutorum muscle; and often its lower side is bounded more or less closely by a sharp adductor ridge, running some way down the valve; this ridge is occasionally almost confluent, in its upper part, with the articular ridge, and in this case sometimes it forms, together with the inflected tergal margin, a large tubular cavity, running up, as in _B. psittacus_ (Pl. 2, fig. 3 _c_), almost to the apex of the valve. Almost invariably there is a slight pit or depression for the lateral depressor muscle; sometimes within the depression there is a little ridge, as in _B. perforatus_ and _nubilus_ (Pl. 4, fig. 3 _a_, and Pl. 6, fig. 2 _a_); and in the case of _B. vestitus_, _flosculus_, and _imperator_ (Pl. 8, figs. 3 _a_, 4 _a_), there are regular crests for this same purpose. The rostral depressor muscle is usually attached in a small pit formed by the folding over of the lower part of the occludent margin: in _B. imperator_ (Pl. 8, fig. 4 _a_) there are regular crests for its attachment, and traces of them may be discovered in _B. vestitus_.