In the first part of this series, p. 313, I referred to the recent invasion of the Eastern Baltic by the Mya arenaria, as first pointed out by Dr. Nathorst. Dr. Petersen has called my attention to an important paper by A. S. Jensen, which has apparently been overlooked by English conchologists, and I propose to condense his results, as they are very noteworthy from their geological lessons.

The shell has hitherto been treated as a typical Arctic shell. Thus Crosse and Debeaux write of its original home: “De l'océan Glacial arctique, qui parait être sa véritable patrie, elle est descendue dans les mers du nord de l'Europe, jusques et y compris la Manche etune partie de nos côtes de l'Océan” (Journ. de Conchologie, ser. III, iii, 254, 1863). Gwyn Jeffreys, in describing the shell, says, “The occurrence of this circumpolar shell-fish so near the tropic of Cancer probably indicates the most southern limit in space of the glacial epoch” (Brit, Conch., iii, 65, 66).