_Shell white, rugged: basis in parts imperfectly porose. Scutum with
the articular ridge minute; adductor ridge prominent, forming a deep
pit for the lateral depressor muscle: tergum with an internal patch of
purple; apex produced, purple._
_Hab._--California, Mus. Brit. and Aug. Gould; associated with _B.
glandula_, and attached to wood.
I have seen two specimens of this species, brought by Lady K. Douglas
from California; and two from Monterey, sent me by Dr. Aug. Gould. This
is a very distinct species, coming nearer to _B. porcatus_ than to any
other species: it is also allied to _B. cariosus_. In the basis being
in parts solid or not permeated by pores, it has claims to be placed in
the next section, in which I at one time included it.
_General Appearance._--Shell conical, rugged, sometimes furnished
with sharp longitudinal ribs; dirty white. Orifice not large, oval,
toothed. Radii rather narrow, with their summits oblique, much
jagged. Basal diameter of largest specimen 2.1; height only 1.3 of an
inch.
_Scuta_, broad, with the lines of growth prominent; internally,
articular ridge very little prominent, sometimes hardly developed,
but thick, ending downwards in a small free point. Adductor ridge
prominent, blunt, produced straight downwards, making a deep
longitudinal cavity for the lateral depressor muscle; in some
specimens this cavity is almost arched over, so as to tend to be
tubular, with a short ridge in the middle (Pl. 6, fig. 2 _a_): in
other specimens there is no trace of this tubular structure. _Terga_,
with the apex beaked, beak triangular, dull purple; the longitudinal
furrow is so shallow as hardly to exist. The basal margin slopes
down on both sides, with a nearly equable curvature towards the
spur; hence the spur is broad in its upper part, and narrow at its
obliquely truncated lower end. Internally, there is an elongated dark
purple patch: the shallow articular furrow is of quite remarkable
breadth; the articular ridge is medial, and the inflected scutal
margin is not wide. The internal surface of the spur is formed into
a ridge, which runs a little way up the valve, and is sometimes
partially separated from the spur itself (fig. 2 _c_), making the
basal extremity toothed or double. The crests for the depressores are
pretty well developed.
_Walls_, moderately strong: inner lamina slightly ribbed: the
denticuli on the bases of the parietal longitudinal septa are sharp:
I could not see any transverse septa in the parietal tubes. The
_radii_ are rather narrow; their summits are remarkably jagged and
very oblique; the septa are plainly denticulated on both sides, but
chiefly on the lower side; each septum itself, towards the inner
lamina of the radius, branches and divides: the interspaces are
filled up nearly solidly. The _alæ_ have apparently their summits
less oblique than those of the radii: their sutural edges are finely
crenated. The lower edge of the sheath is hollow underneath. The
_basis_ is flat; it is rather thin, and imperfectly porose; in parts
it is not at all porose, in others it is traversed only by very
minute pores: there is nevertheless, in some parts, even where the
upper layer is not porose, an underlying, cancellated layer.
_Animal's body_ unknown.