Shell thin, translucent, white, very slender, elongated, narrow, fusiform, with a long, narrow, tapered, nearly straight canal, and a tall, gradually tapered, acute spire. Whorls eight, evenly rounded, but not very convex, with a distinct, flattened, smooth subsutural band. Suture well marked, but not deep, decidedly oblique. Surface everywhere covered with conspicuous, regular, raised, revolving cinguli, in some parts with one or more smaller revolving lines in the spaces between them. The cinguli are obtusely rounded and entirely smooth, as well as the spaces between them, which are of about the same breadth ; on the penultimate whorl there are about fifteen cinguli. and on the upper whorls five or six. The large, acute, brown nucleus consists of about four and a half whorls, which increase regularly ; the apical whorl is small, rounded and prominent; the others are distinctly carinated and shouldered; the portion above the shoulder slopes at a wide angle and is a little convex and nearly smooth, except close to the carina ; the part below the carina of the shoulder is flattened and nearly straight, or even narrowed toward the suture, and crossed by regularly spaced, thin elevated transverse ribs, with wider intervals; these ribs extend a little above the carina and then fade out; they run nearly straight across all the whorls, except the first two, where they are more or less oblique; there is usually, on the larger whorls, a raised revolving line, or small carina, just above the suture. The aperture is very elongated and narrowed at the base of the canal, which is much prolonged and slender, a little curved, owing to a slight sinuous curvature of the columella-margin. The posterior notch in the outer lip is rather deep and narrow, situated immediately at the suture. The subsutural band, corresponding to it, shows faint curved lines of growth, parallel with its margin. Color white, except the nucleus, which is pale chestnut-brown. Length of one of the largest specimens, 23 mm.; breadth, 6.5 mm.; height of spire, l1 mm. length of aperture and canal 11.5 mm.; breadth of aperture, 3 mm.

R catharinae is characterized by the larval shell, its very slender outlines and comparatively strong spiral sculpture. F. mirmidina has more convex whorls, no spiral sculpture and a very characteristically curved columella. F. monoceros differs by having less distant axial ribs on the larval shell and much fainter sculpture.