MLA

Co, Clason Map. Map of City of Helena, Montana / Copyrighted 1909 by Geo. S. Clason. Clason Map Company, 1909. https://collections.library.yale.edu/catalog/15814925.

APA, 6th edition

Co, C. M. (1909). Map of city of Helena, Montana / Copyrighted 1909 by Geo. S. Clason. [Maps]. Clason Map Company, https://collections.library.yale.edu/catalog/15814925.