Plains Indian society, despite its lack of lineage and clan, still has a social structure. This structure is "horizontal" or generational in character and has little depth. The extended family groupings in terms of matrilocal residence or centered around a sibling group are amorphous but flexible. The bilateral or composite band organization, centered around a chief and his close relatives, may change its composition according to various circumstances--economic or political. The camp-circle encompasses the tribe, provides a disciplined organization for the communal hunt, and a center for the Sun Dance and other tribal ceremonies which symbolized the renewed unity of the tribe and the renewal of nature. The seasonal alternation between band and tribal camp-circle is related to ecological changes in the environment, and particularly to the behavior of the buffalo ... The working hypothesis proposed earlier ... that "tribes coming into the Plains with different backgrounds and social systems," can be tentatively extended to Plains social structure as a whole, despite the variations noted. That this is in large measure an internal adjustment to the uncertain and changing conditions of Plains environment--ecological and social--rather than a result of borrowing and diffusion, is still highly probable. The loose bilaterality seen by Eggan as a characteristic of Plains society was part of the earlier Shoshone social background. But more centralized political units and predominantly "horizontal" organizations, such as age-grade and soldier societies, were more weakly developed than among many other tribes of the northern Plains. This fact, as well as the acquisition of firearms by the northern tribes, may have been responsible for the manifest military weakness of the Shoshone and their westward retreat beyond the Rocky Mountains. BIBLIOGRAPHY _Abbreviations_ AA American Anthropologist AMNH-AP American Museum of Natural History, Anthropological Papers. New York BAE-B Bureau of American Ethnology, Bulletin CD Congressional Document, Washington MPUS Report of the President of the United States. Washington RCIA Report of the Commissioner of Indian Affairs. Washington RSI Report of the Secretary of the Interior. Washington RSW Report of the Secretary of War. Washington UC University of California Publications. Berkeley and Los Angeles -AAE American Archaeology and Ethnology -AR Anthropological Records Alter, J. Cecil 1925. James Bridger. Salt Lake City, Utah. Ballard, D. W. 1867. RCIA 1866, CD 1284, pp. 190-192. 1869. RCIA 1868, CD 1366, pp. 656-659.