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.