MLA

Pearson, Norman Holmes, 1909-1975. Matthiessen, Peter. Box 77. undated. https://collections.library.yale.edu/catalog/17260077.

APA, 6th edition

Pearson, Norman Holmes, 1909-1975. (["Box 77"])(undated)[Matthiessen, Peter]. https://collections.library.yale.edu/catalog/17260077.