_Shell purplish dark brown: inner lamina of the parietes cancellated.
Scutum finely striated longitudinally. Tergum with the longitudinal
furrow shallow and open; basal margin on both sides sloping towards the
spur._
_Hab._--West Coast of South America. Mus. Cuming.
_General Appearance._--Shell conical, with a large, rhomboidal
orifice; walls rather thin, coloured, together with the radii and
operculum, dark purplish-brown; sheath nearly colourless. Walls
smooth, slightly irregular, very finely striated longitudinally.
Basal diameter of largest specimen .8 of an inch.
_Opercular Valves_, unusually smooth, that is without prominent
growth-ridges. _Scuta_, finely striated longitudinally, with the
sharp striæ closely approximate. The teeth on the occludent margin
are sharp, and stand some way apart from each other. Internally, the
whole surface is remarkably flat and smooth: the articular ridge is
of moderate breadth, and slightly reflexed: there is no adductor
ridge, and the oval depression for the lateral depressor muscle is
extremely slight. _Terga_, with the longitudinal furrow very slight;
the bottom of this furrow is feebly striated longitudinally, and
there is a trace of a fine, rounded ridge on the scutal margin, as in
_B. decorus_. The basal margin slopes on both sides towards the spur,
which is of moderate length and breadth, with its lower end truncated
and parallel to the carino-basal margin; the spur stands at about
once and a half its own width from the basi-scutal angle. Internally,
the valve is lined by very dark, purplish-brown corium; the articular
ridge is prominent; in the upper part of the valve, parallel to the
articular ridge, there are two or three feeble ridges; there are no
crests for the tergal depressores.
The _Parietes_, though moderately thick, yet are light and fragile;
the denticuli at the bases of the longitudinal septa are prominent,
and those on the adjoining septa are united together, making a
network (Pl. 2, fig. 7 _d_), but the interspaces between them are not
filled up by solid calcareous matter (as is the case with every other
species of the genus), but are only crossed at successive levels by
fine transverse calcareous septa; the internal lamina thus becoming
cancellated, and, though thick, fragile. Hence, in a transverse
section of the parietes, the ordinary parietal tubes or pores are
seen to be lined on their inner sides by five or six rows of very
minute pores. I have not seen any other instance of this structure.
The internal lamina is ribbed, as usual, on its inner surface, by
the projection of the longitudinal septa. The ordinary parietal
tubes are open, to nearly the summit of the shell. The _radii_ are
rather thin, and unusually fragile; their summits are parallel to
the basis: their septa, as seen on the sutural edges, are extremely
thin and denticulated on both their upper and lower surfaces, on the
side towards the internal lamina: towards the external lamina, the
septa are simple, and the small square pores thus formed, are open
or not filled up. The _alæ_ have their summits extremely oblique,
being added to very little during the diametric growth of the shell;
the narrow margin, however, which is thus added, is coloured red,
the rest of the sheath being nearly colourless: the sutural edges
of the alæ are smooth. The _basis_ has a thick, underlying, finely
cancellated layer of shell.
_Animal's body_ unknown.
A young specimen, .2 of an inch in basal diameter, differed from
the above in being of a much paler purplish-brown. This species is
distinct from all its congeners, in its peculiar colour, and likewise
in the structure of the inner lamina of the parietes. As already
stated, it comes nearer to _B. decorus_ than to any other species.