We will now try and picture to ourselves how the circulation of the water was affected by the breach in the land bridge. We have seen in the earlier part of these papers that one of its effects was that the southern and western part of the Baltic became rapidly richer in marine forms. This is because the Straits between Gjedserodde in the island of Falster and Darrserort on the mainland of Mecklenburg form a great barrier to the eastern migration of the marine mollusca, whose species increase greatly in numbers when we pass westward of them. This seems to again point to the fact that the inflow of salt waters into the Baltic from the North Sea passes chiefly through the deeper Belts and not through the shallower Sound, which is the chief outlet of the more brackish Baltic water. On the other hand, the Swedish side of the sea remains poor in fauna until we reach the latitude of the island of Saltholm, due partly to its greater shallowness, which only allows a smaller proportion of the incoming North Sea water to pass.