The commonest and most stable position of empty valves of large pelecypods on the wave-washed zone of beaches is concave down; this position is due to the presence of fast currents and the absence or inadequacy of bioturbation. In contrast, most large pelecypod valves on the surface of the continental shelf lie in a concave up position in response to the action of carnivores and scavengers and to powerful bioturbation and the absence of fast currents. These observations of preferred position can be applied to paleoecological studies of ancient strata.

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