Modiolarca tumida is a small bivalve up to 2 cm in length. The valves of the shell are equal in size, shape and depth and both have a swollen, plump appearance. The shell is broadly oval in outline. The shell is yellow with ribs in the anterior and posterior regions. The periostracum is light green, with blotches of brown, red-brown or purple. The margin of the shell is smooth but crenulate where it meets the ribs. The inside of the shell is bright white.