The following pages have a twofold purpose. First, there is extended to new territory an analysis of aboriginal population and ecology in California which has already encompassed the San Joaquin Valley (Cook, 1955) and the north coast (Cook, 1956). The area treated here is a portion of that occupied by the Costanoan linguistic division, which extended from San Francisco Bay throughout the interior ranges and along the coast as far south as the latitude of Salinas and Monterey. However, in view of the many accounts which have been written concerning the settlement of coastal California and the establishment of the missions, it seems preferable to devote attention almost exclusively to one restricted region and to deal with this as exhaustively as possible. The area selected embraces the east shore of San Francisco Bay and its hinterland, including what is now Alameda and Contra Costa counties. It is quite true that some of the tribal groups inhabiting this territory may not have been members of the Costanoan stock. On the other hand, in their relation both to the native environment and to the invading white man their activity conformed in all important ways to that of their bona fide Costanoan neighbors. Hence it is proper to treat all the aborigines in the area on a common basis.