_Shell white, longitudinally folded; radii narrow. Scutum internally
without an adductor ridge._
_Fossil_, Sub-Appennine formations; Colle in Tuscany; Mus. Greenough.
I have seen only two specimens of this species attached to rock,
collected by Mr. Greenough, at Colle, and kindly given by him to me.
The species comes near to the living _B. crenatus_, also found fossil
in deposits of this same age; it differs, however, distinctly from
that species, in having its basis permeated by pores, and, in a less
degree, in the sutural edges of the radii being more plainly crenated:
the opercular valves of the two species closely resemble each other.
This may be the _B. stellaris_ of Bronn, but it is futile attempting to
identify the species of this genus merely by external characters, even
when aided, as in this case, by an excellent drawing of the shell.
_General Appearance._--Shell conical, with broad rounded longitudinal
folds; orifice of moderate size, oval; radii narrow, with their upper
margins oblique; but the summits of both specimens had been much
broken. Colour, as it appears, originally white. Basal diameter of
largest specimen 3/4 of an inch.
_Scuta_, with the upper portion much reflexed; the articular
ridge is very prominent, and the articular furrow of great width;
when the valve is viewed from the outside the articular ridge is
very conspicuous: there is no adductor ridge. _Terga_, with the
longitudinal furrow very slight; the spur is from one third to
one fourth of the width of the valve, and its basal end is blunt
and almost truncated; it stands about half its own width from the
basi-scutal angle. Internally, the articular ridge is very prominent,
and the articular furrow narrow and deep, extending down the valve in
the line of the spur.
_Parietes_: the parietal tubes are remarkably large, and I think
this can hardly be an individual peculiarity: the tubes are crossed
by many transverse septa, close down to the basis. The _radii_ are
narrow, and have jagged, oblique summits: their sutural edges have
very distinct septa, barely denticulated, with the interspaces filled
up solidly. The _alæ_ have oblique summits; I was unable to make out
the structure of their sutural edges. The _Basis_ is very distinctly
permeated by pores, which are crossed by transverse septa.
The shell and opercular valves of _B. corrugatus_ so closely resemble
the same parts in _B. crenatus_, that I should not be much surprised at
seeing the two species graduating into each other, if a larger series
of specimens, from beds intermediate in age between the Sub-Appennine
formations and the present time, were obtained. If indeed the basis of
_B. crenatus_ were permeated by pores, the two species could hardly be
discriminated.
_Section_ D.
_Parietes permeated by pores. Basis and Radii not permeated by pores._