MLA

The Hydra [Graphic]. publisher not identified, 0AD. https://collections.library.yale.edu/catalog/10713538.

APA, 6th edition

The Hydra [graphic]. (0 C.E.). [Caricatures and cartoons, Periodical illustrations, Satires (Visual works) England 1770, Etchings England London 1770]. publisher not identified. https://collections.library.yale.edu/catalog/10713538.