A number of illustrations are presented, which show some characteristics of the reefs occurring in the Upper Visby and Hogklint Beds of Gotland. These series should probably be correlated with the uppermost Llandoverian and Lower Wenlockian. Reef growth took place as the water became progressively shallower. There are no reefs in the lower part of the Upper Visby Beds, which consist of marl with limestone Ienses, and was formed in relatively deep water. No reefs occur in the uppermost Hogklint, which consists of an algal limestone formed in very shallow water.