page 145 note * The inference that the Forest-bed occupies an interglacial position is strengthened by the evidence of certain marine deposits which immediately overlie it. These (known collectively as the Leda-myalis bed) occur in irregular patches, which, from the character of their organic remains, cannot all be precisely of the same age. In one place, for example, they are abundantly charged with oysters, having valves united, and with these are associated other species of molluscs that still live in British Seas. At another place no oysters occur, but the beds yield two Arctic shells, Leda myalis and Astarte borealis, and some other forms which have no special significance. Professor Otto Torell pointed out to Mr Reid that these separate deposits could not be of the same age, for the oyster is sensitive to cold and does, not inhabit the seas where Leda myalis and Astarte borealis flourish. From a consideration of this and other evidence Mr Reid concludes that it is possible that the deposits indicate a period of considerable length, during which the depth of water varied and the climate changed. Two additional facts may be noted: Leda myalis does not occur in any of the underlying Pliocene beds, while the oyster is not found in the Weybourn and Chillesford Crag, though common lower down in the Pliocene series. These facts seem to me to have a strong bearing on the climatic conditions of the Forest-bed epoch. They show us that the oyster flourished in the North Sea before the period of the Weybourn Crag—that it did not live side by side with the Arctic forms of that period—and that it reappeared in our seas when favourable conditions returned. When the climate again became cold an Arctic fauna (including a new-comer, Leda myalis) once more occupied the North Sea.
