MLA

Grob, T. (Trautman), artist. Execution of James P. Casey & Charles Cora by the Vigilance Committee of San Francisco Thursday May 22d, 1856. [1856?]. https://collections.library.yale.edu/catalog/2015923.

APA, 6th edition

Grob, T. (Trautman), artist. ([1856?])[Execution of James P. Casey & Charles Cora by the Vigilance Committee of San Francisco Thursday May 22d, 1856]. https://collections.library.yale.edu/catalog/2015923.