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.