The oldest Mesozoic reefs appear to have been formed upon the same plan as those which succeeded them through the consecutive geological ages; and all had the same general grouping of families, genera, and shapes which prevails in every modern reef. The scheme, which was feebly represented in the oldest reefs, became more fully developed in the Oolitic period, and was as perfect during the Nummulitic age as it is now. The faunas of the consecutive reefs rarely had species in common, but the genera were most constant and persistent. Certain coral-shapes and methods of growth and of reproduction prevailed during the whole of the periods; and there is a fair inference to be drawn that the external physical conditions which now are absolutely necessary for the formation and persistence of great aggregations of corals were present during the Mesozoic and Cainozoic periods. These physical conditions have a geographical importance; and the same disposition of islands, deep and shallow seas, with remote continental spaces and rivers, which now prevails in the coral-areas, must be suggested to have existed in the reef-bearing seas of the past. The simplicity of the physical conditions, and their continuance on some one area during all ages, offer an explanation why the species and genera of the reefs and deep seas are respectively so representative and so evidently formed upon one definite plan. There are relations, which can only be hinted at in this communication, between the floras and the reptilian and mammalian faunas of present