The occurrence of Mya arenaria in fossil deposits of Western Europe has long been a subject of much controversy and discussion. So long as only rare remains in scattered localities were known, the autochthonity of the strata was questioned. Later, when it became obvious that the shells belonged to layers of undisputed age, doubt arose as to the correct identification of the fossils, the current opinion being that we had not to do with genuine Mya arenaria L., but with a specifically different form: Mya pseudarenaria Schlesch (= M. truncata ovata Jensen). The true Mya arenaria would, according to this view, not have reached Western Europe until the atlantic, perhaps even subboreal, phase of the Holocene, at any rate dating after the formation of the Strait of Dover. A modification of the latter opinion was upheld by a few authors who acknowledged the presence of true Mya arenaria in Pliocene and in Holocene deposits, but its absence in the period between, and who concluded that there must have been two waves of development in the ancestry of the species: a Pliocene one which became extinct towards the Glacial period and a Holocene one beginning after the transgression of the Strait of Dover, a large gap, devoid of Sand-gapers, extending during the greater part of the Pleistocene. Both migrations were assumed to have taken origin in the East coast of the U.S.A. While the fossil state of Mya arenaria has been in the dark for a long time we are much better informed of its recent distribution. It is a boreal shell¹), occupying the littoral zone from the Norwegian coast to the West coast of France in the Eastern Atlantic, and again in the Western part of the Mediterranean. In the Baltic Sea it penetrates even into the Bothnian Gulf. In Holland it is abundant along the North Sea coast, especially in the Waddenzee and in the estuaries of the provinces of Zuid-Holland and Zeeland. In the Western Atlantic it stretches from Greenland to Florida. Even the North Pacific is inhabited by a race of Mya arenaria.