MLA

Riola, Henry. Key to the Exercises of the Manual for Students of Russian; Based upon the Ollendorffian System of Teaching Languages ... Houghton, Osgood and Company, 1878. https://collections.library.yale.edu/catalog/30880812.

APA, 6th edition

Riola, H. (1878). Key to the exercises of the manual for students of Russian; based upon the Ollendorffian system of teaching languages ... Houghton, Osgood and Company. https://collections.library.yale.edu/catalog/30880812.