Adults of this species are small to moderately large brachiopods, with a maximum shell width of 1.18 in (30 mm). The shell is roughly oval in shape, and the anterior commissure is slightly grooved. Externally, the shells have numerous and somewhat coarse radial ribs. These ribs become knobby, forming smooth, rounded tubercles (nodules) at the rear and side margins of both valves, although they are most developed on the sides of the ventral beak or umbo. The beak represents the initial point of growth of a valve. A wide number of concentric growth lines are present. Shell color ranges from white to yellowish gray. The small del-tidial plates are separated. The species is endopunctate, which means that there are tiny canals called endopunctae extending from the inner surface of the valve almost to the outside. In T. retusa, the endopunctae are usually arranged radially (corresponding to the external ribs) on the inside of the valves. Spicules, or hard calcareous plates, are found in the body tissue near the lophophore as well as in the tissue covering the mantle canals. The spicules help to support the soft body tissues. The tip of the pedicle divides into short rootlets that are able to penetrate the substrate.
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