Davidson thus describes the external characters of this species in his Sil. Suppl. pp. 101, 102. The description is practically the same as that which he previously gave in his Sil. Mon. “Marginally of a somewhat elongated, pentagonal shape, truncated, and slightly indented in front, broadest about the middle. Tapering posteriorly. Valves almost equally convex, with a slight median depression near the front in the ventral valve. Beak strongly incurved, and truncated by a minute oval foramen; surface smooth, marked by a few concentric lines of growth. Length 6, breadth 5 lines, but the generality of specimens are much smaller.” Davidson refers to the spirals of this species both in his Sil. Mon. and in his Sil. Suppl. Sil. Mon. p. 115: “I had noticed the presence of spirals in 1847, and recorded the observation in the ‘Bulletin Soc. Géol. France,’ and Lindström has also detected them in a Gothland specimen.”