The valves are thin, relatively shallow, and almost oval in shape. They can reach 6.4 cm (2.5 inches) long. Individuals can achieve a maximum weight of 45 grams. The exterior of the valves is covered in a shiny brown periostracum, hence one of its common names, "varnish clam". The periostracum is often worn away at the umbo. There are concentric rings on the outside of the valves which can be observed where the periostracum has been removed. The interior of the valves is deep purple to almost white in some individuals. The prominent hinge ligament is external to the valves. Nuttallia obscurata has a large white, muscular foot that it uses to dig in the sand and for propulsion. The animal has two long, white, separate siphons. Nuttallia obscurata can live at least six years. They begin life.
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