This résumé deals only with those phases of Mesozoic history which have a direct bearing on the biologic, relationships of North and South America by indicating on the one hand epochs of probable marine connection between the Pacific and the Atlantic through tropical America, and on the other hand epochs of probable land connection between North and South America. The evidence to be presented consists of the distribution of the sediments of different epochs on the present land area, their character as marine or continental, and the relationships of the marine faunas found in them. Most of the data are recorded on the Geologic Map of North America compiled by Willis and Stose, and in the literature abstracted by Willis in his Index to the Stratigraphy of North America. These have been supplemented by the consideration of more recent papers by C. Burckhardt, E. Böse, G. R. Wieland, J. P. Smith,. . .