Shell is thick, slightly inflated to compressed and elongate; generally twice as long as wide; ventral margin either straight or slightly curved in juveniles. Umbo is low and rarely raised above hinge line. Epidermis is greenish-brown with faint green rays, dark brown to black with age. Inside shell beak cavity shallow to absent; pseudocardinal teeth triangular and streaked; lateral teeth short and straight; nacre (lining) purple, occasionally pink or white.
Similar species: Young or small spectaclecase could be confused with the spike, but they appear pinched. The black sandshell is longer and thicker with a more prominent posterior ridge and a shiny exterior that is usually darker.