At a site which was subsequently destroyed by construction work at the Oahe Dam on the main stem of the Missouri river in South Dakota, the remains of both rectangular and circular structures were found. Two types of rectangular houses were built ; stratigraphie evidence shows that one developed out of the other. Last to be erected were the circular structures. The rectangular houses have been identified as Mandan, while the circular belonged to the Arikara culture. The location apparently had been abandoned for some time by the Mandan before the Arikara moved in. The aerial photograph illustrates how some of the overburden was removed before digging in the house remains was started. In the Missouri Basin complete floor plans have been obtained in the process of uncovering former village areas. The molds left by wall posts outline the shape of such structures. Cache and fire pits occur at various places within the area of the houses. Spring 1951 27