Fossil in Coralline Crag, Ramsholt, Gedgrave, Sutton; Mus. S. Wood, Bowerbank, J. de C. Sowerby. Rauville, dans le Cotentin, Mus. G. B. Sowerby. Var. plicatus, Coralline Crag, Sutton, Mus. S. Wood. Bolderberg, near Hasselt, Belgium, Mus. Bosquet.
General Appearance.—Shell conical or tubulo-conical, often rather globose; walls frequently thin, either very smooth, or deeply plicated longitudinally: occasionally the same specimen is smooth in the upper part, and strongly plicated in the lower. The Radii in the larger specimens are wide, and with their upper margins only slightly oblique; in the smaller, they are narrower and much more oblique, but in each case their upper margins are smooth and slightly bowed. Colour apparently originally nearly white, but with the alæ generally, in the smaller specimens, clouded with a dark tint: the radii are usually striped feebly in longitudinal lines. Basal diameter of largest specimen .8 of an inch; but this seems to have been an unusual size.
Scuta: narrow, with the basal margin forming an unusually small angle with the occludent margin; surface slightly convex, with lines of growth approximate, moderately prominent; on the tergal half of the valve, two distinct rather broad furrows, with sometimes a third, and even a fourth, nearer to the occludent margin (Pl. [8], fig. [6 a]), extend from the apex down the valve, and give it a very peculiar appearance: the furrows near the tergal margin are the deepest. Internally, the upper part of the valve is roughened with small points: the articular furrow is unusually wide: the articular ridge is very prominent and but little reflexed, with the lower end almost abruptly cut off: the adductor ridge is prominent, but short: there are small deepish pits for the rostral and lateral depressores.
Terga ([6 b]), broad, flat, with a slight narrow prominent rim along the scutal margin, which margin is slightly bowed. The basal margin on the carinal side of the spur slopes so gradually towards the spur, that the latter is barely distinct, and is very short, not depending nearly half its own width beneath the basi-scutal angle: it is broad, namely, measured across the upper part, as broad as half the valve; its basal end is obliquely rounded off on the carinal side; it is placed close to the basi-scutal angle. The carinal margin of the valve is just perceptibly bowed, and is formed by rectangularly upturned lines of growth. Internally, the upper part of the valve is rough; the articular ridge is prominent; the crests for the tergal depressor muscles are moderately well-developed.
Parietes, not porose; internally, the ribs are smooth, with their basal edges very finely or barely denticulated. The radii (as already stated) are of variable breadth; they have their upper margins either very slightly or highly oblique, but always smooth and rounded: their sutural edges are quite smooth, or sometimes, with a strong lens, traces of transverse striæ, representing septa, can just be discovered. The alæ have their upper margins very oblique; their sutural edges are, in the large specimens, quite smooth; in the younger ones, plainly crenated; the recipient furrow being clearly marked with these teeth. Basis plainly porose.
Varieties.—It is certain that there are longitudinally plicated specimens of this species, and that the obliquity of the upper margins of the radii also varies a little: nevertheless some of the deeply plicated specimens undoubtedly have a very different aspect from the ordinary varieties, and do really differ in the sutural edges of the alæ being crenated, and in the greater narrowness and obliquity of the radii; but these points are all commonly variable. I have not seen any large specimens of the variety, plicatus, so as to compare them with the large specimens of the normal form, yet I can hardly entertain any doubt, considering their agreement in so many important points, that I have rightly treated these forms as mere varieties; it is unfortunate that none of the specimens of the var. plicatus seen by me have had opercular valves, as their presence would have removed all shadow of doubt.
Affinities: this is a strongly characterised species, and nearly allied only to the following species, [B. dolosus]. The furrows on the scuta in some degree resemble those on [B. lævis], but there is no alliance with that species. It is certain that amongst recent species, the chief affinity is with [B. Hameri] and [amaryllis].