Shell reaching a total length of 4 – 7 cm (rarely up to11 cm). The height of the shell is usually less then half the length. The shape of the shell is ovate-oblong and swollen. The lower margin is often forming a straight line in the centre and is equally curved up to the posterior- and anterior ends. The anterior end is slightly tipped down. Occasionally the lower margin can be curved inwards, which gives the shell the shape of a “kidney”, to be compared with the shape of the shell from the Freshwater pearl mussel (Margaritifera margaritifera). The Thick shelled river mussel can though be distinguished from the Freshwater pearl mussel by its larger radius (thickness) at the centre. The valves are very thick and heavy. The colour is dark green-brown-black. Its surface is often covered with a layer of lime deposit or an iron- manganese coating. The umbonal rugae are strong and consist of close wavy rows with ridges. In larger specimens the rugae are mostly lost by corrosion. Powerful hinge-teeth are present. The anterior lateral and cardinal teeth are well developed and in the left valve they form a line. In the right valve the anterior lateral tooth is strongly pronounced and shaped like a very big triangular wedge.
Note that there is a possible risk of confusion between the thick shelled river mussel and the freshwater pearl mussel, but the different shape of the hinge-teeth is a distinct feature when separating the two species.
Photo: Håkan Holmberg, Scientific author: Ted von Proschwitz