MLA

Cross, W. R. (William R.). Pine Ridge Agency. Indians. Folder 9. circa 1891. https://collections.library.yale.edu/catalog/2001106.

APA, 6th edition

Cross, W. R. (William R.). (["Folder 9"])(circa 1891)[Pine Ridge Agency. Indians]. https://collections.library.yale.edu/catalog/2001106.