« Shell small, rather thin and laterally compressed, elongate, strongly inequilateral; roughly quadrangular in outline, with more or less parallel, straight dorsal and ventral margins and small, pointing umbones at anterior end of valves. Anterior margin very small, strongly arched, not extending forward beyond the umbones. Posterior margin broadly and evenly rounded. Ventral margin often slightly concave medially, decidedly bent dorsalward near anterior end. Posterodorsal angle obtuse, but well marked, anterior to mid-length of shell. Outer surface with a more or less distinct low rounded ridge […] radiating obliquely towards posteroventral end of valves. Sculpture of numerous, fine concentric growth lines, often becoming somewhat irregular and with a few transverse low wrinkles towards the posterodorsal angle. Lunular ridges hardly visible, sometimes forming a small bulge in front of the umbones. » – Ozturk & al.: “On the occurrence of Mytilaster marioni (Locard, 1889) in Bafa Lake”, Hydrobiologia vol. 485(1), 2002, p.126-127. The species competes with the opportunistic Arcuatula senhousia (Benson in Cantor, 1842), and is often replaced by the latter in stenohalin shallows.