_Shell dull red, freckled with white. Scutum internally without an
adductor ridge; tergum with the spur, sharply truncated, almost one
third of width of valve._
_Hab._--West coast of South America, Mus. Cuming; attached to an
Avicula.
The appearance of the fragile shell, in the one group of specimens
which I have seen, leads me to suspect that they may have grown under
unfavorable circumstances. This species differs considerably in general
aspect, but not much in essential characters, from _B. amphitrite_; the
absence, however, of an adductor ridge to the scutum, and the sharply
truncated spur of the tergum, are sufficient to distinguish them. In
the opercular valves this species comes near to _B. vinaceus_, also
from the west coast of South America; but the striated scuta of that
species, the cancellated inner lamina of the parietes, the general
colouring, and square porose radii, are amply diagnostic characters.
_General Appearance._--Shell fragile, tubulo-conical, orifice large,
passing from diamond-shaped into oval. Colour fine dark rose,
freckled with transverse, sharply pointed, fine zig-zag white lines:
the pink is also so arranged as to obscurely give to the walls a
longitudinally striped appearance: radii generally rather whiter than
the walls, and similarly freckled: terga similarly freckled: scuta
dull red, with a white band along the scutal margin. Basal diameter
of largest specimen half an inch.
_Scutum_, externally smooth: internally, articular ridge moderately
developed, slightly reflexed: there is no adductor ridge: there is
a distinct pit for the lateral depressor muscle. _Tergum_, with the
scutal margin unusually prominent, toothed: longitudinal furrow
shallow, the edges apparently having no tendency to fold in: spur
short, barely one third of width of valve, with the lower end sharply
truncated, parallel to the basal margin: articular ridge and crests
for the depressores moderately prominent.
_Compartments._--Walls very fragile, with the outer lamina not
thicker than the inner lamina. _Radii_ fragile, broad, with their
summits moderately oblique; their sutural edges have the septa
plainly denticulated on both sides, with the interspaces filled up
solidly nearly to the tips of the septa. _Alæ_, with their summits
more oblique than those of the radii; their sutural edges smooth.
_Basis_ with an underlying cancellated layer. _Mouth_: labrum with
three unusually large teeth on each side of the notch: mandibles with
the fourth tooth tolerably well developed, the fifth being confluent
with the inferior angle. _Maxillæ_ simple. _Cirri_,--first pair with
one ramus longer by about four segments than the other ramus, which
has considerably protuberant segments: second pair with segments only
moderately protuberant: sixth pair with segments much elongated, but
bearing only four pairs of spines.