Box-folder 175:10

Scope and Contents From the Sub-Series: The Reference and Citation Files were retained in Tombaugh's original order, however empty folders were removed. These files contain reprints, photocopies, notes, and citations for articles and books; in addition, card files contain citations to these and other sources. Tombaugh's original list of categories is filed at the beginning of the Reference and Citation Files, followed by the subject files arranged alphabetically, with the card files placed at the end.