Correspondence sent: This series is arranged chronologically. Most of the letters were written by Hasselborg to his family. There is one letter written to Hasselborg which was probably enclosed with one of his letters; it remains with this series. Hasselborg sometimes wrote letters on the backs of letters he received. Those letters written to Hasselborg are not numbered in this series.

Correspondence with Museum of Vertebrate Zoology, University of California, Berkeley, as well as annotated book and items stored in book