_Radii with their upper margins oblique and smooth; sutural edges
smooth: basis permeated by large pores. Tergum with the spur not very
short, broad as one third of valve._
_Fossil_ in Red and Mammaliferous Crag, England; Mus. S. Wood,
Bowerbank, Lyell, J. de C. Sowerby, Henslow, &c. Mammaliferous Crag,
Postwick, near Norwich, Mus. Lyell.
This species so closely resembles _B. bisulcatus_, both externally and
in all the essential characters of the parietes, radii, and basis,
that it is quite superfluous to describe over again these parts. The
specific characters are derived from the opercular valves, which
present well defined distinctions, found by me constant in several
specimens of both species. _B. dolosus_, like _B. bisulcatus_, has
quite smooth and deeply plicated varieties, often adhering to the same
univalve. The ribs on the inner surfaces of the parietes are remarkably
prominent. I think the upper margins of the radii are in this species
rather more oblique than in _B. bisulcatus_. The sutural edges of the
radii are marked by the finest striæ, representing septa. The sutural
edges of the alæ are generally distinctly crenated. The basis is often
slightly cup-formed, and very plainly porose: its surface is marked by
radiating ridges. The orifice of the shell is large, and elongated,
especially in young specimens. The basal diameter of the largest
specimen is .4 of an inch.
The _Scuta_ have no trace of the two or three longitudinal furrows so
conspicuous on these valves in _B. bisulcatus_, and which, in that
species, run down from the apex of the valve; this fact showing that
the furrows occur in quite young individuals. The whole valve is not
quite so narrow as in _B. bisulcatus_, but otherwise agrees with it
in shape: internally, there is hardly any difference: the articular
furrow is not so wide: the articular ridge is very prominent, and
abruptly truncated at its lower end: the adductor ridge is also
prominent; it here runs a little higher up the valve than in _B.
bisulcatus_. The _Tergum_ differs more in the two species: the spur
is not so broad; measured in its upper part, it is only about one
third of the entire width of the valve, instead of being half as
wide as the valve: it is considerably longer, depending beneath the
basi-scutal angle more than half its own width: the basal margin of
the valve on the carinal side, does not slope so gradually into the
spur: the occludent and carinal margins are slightly arched, as in
_B. bisulcatus_. Internally, the surface is rough, the articular
ridge is prominent, and the crests for the tergal depressores are
well developed,--all as in _B. bisulcatus_. It is remarkable, how
generally the opercular valves have been preserved in this species
in its fossil condition, as compared with most other species of the
genus.
It is not easy to distinguish by external characters the rugged
varieties of this species from _B. crenatus_; indeed, the only
difference is that the furrows receiving the edges of the radii,
generally, exhibit in _B. crenatus_ a slight impression of the septa,
which are entirely absent in _B. dolosus_. By internal characters,
such as the non-porose parietes, and porose basis, our present species
widely differs from _B. crenatus_.