Shell rhombic-oval, rather gibbous, thin, semitransparent, glossy and prismatic. Sculpture of numerous and close-set delicate, microscopic concentric striae. Posterior and anterior lateral teeth are about equally developed. In the right valve they are short, triangular, slightly inclining inwards, not widely separated; in the left valve long, erect, laminar, and parallel with the hinge-line. Distinguishable from Mysella bidentata by being narrower, not depressed, but convex, having a glossy surface, and by the posterior side being disproportionately small and abruptly truncated; that side in M. bidentata is invariably squarish, and more or less angulated. The teeth in the right valve of M. tumidula are much smaller than in the same valve of M. bidentata, and are not so widely separated; they are triangular and not leaf-like, and slightly incline inwards instead of being erect. Adductor scars are conspicuous, irrerularly oblong (M. tumidula).
Smaller than Mysella bidentata (<3 mm long).
Pale yellowish-brown. Inside iridescent and polished, very finely marked with slight lines which radiate from the beaks.
In muddy ground.
Only known in the North Sea from northern parts (Distr. M. tumidula).