(Smith)
Description (shell):
Shell is brittle, globular, equivalve, with small mesoplax, its upper surface divided into two triangular plates. Inequilateral, umbones are near the midline. Circular in outline except for right-angled inset of the permanent anterior pedal gape; a posterior gape is present which does not interfere with the outline. Umbonal reflection small, raised above the umbones anteriorly, overgrown by the mesoplax. Ligament is small, with curved chondrophore in left valve, small attachment area below umbo in right valve.
Sculpture as follows:
1. Anterior lobe normally with less than 50 fine serrated ridges all running parallel to the upper margin of the pedal gape and mostly widely spaced; also with a clearly differentiated anterior area or lunule where it curls up to meet the antero-lateral border of the mesoplax.
2. Anterior disc with similar but more closely spaced ridges running obliquely.
3. Median disc, a shallow groove with ridged margins and concentric lines.
4. Posterior disc, smooth with concentric lines.
Growth stages are not clear. There are no apophysises. Interior of shell with the groove of the median disc showing through. Anterior adductor scar is thin, smooth, on the umbonal reflection. Posterior adductor scar is large, oval and striated. Pallial line is wide, the sinus occupying the greater part of the shell. Margin is smooth.
Size:
Up to 19 mm in length.
Colour:
White. Periostracum olive. Interior of shell white.
Habitat:
On drifting timber.
Distribution:
A rare species known only from wood collected off the coasts of Northumberland and Durham (Distr. X. praestans).