Salix hesperia (Knowlton) Condit and Salix inquirenda Knowlton are Miocene species with an interwoven taxonomic history. Salix hesperia was first described as Juglans hesperia by Knowlton (1898). Brown (1937) included it in S. inquirenda, together with S. remotidens, hereby disregarding the priority of the epithet hesperia. Condit (1944) corrected this error and united all three species under the name S. hesperia. Axelrod (1992) reestablished S. inquirenda as a separate species, but did not give a formal description. Based on revisions of 105 specimens, the present study introduces new distinguishing characteristics, including overall leaf shape, base and apex form, length to width ratio, venation pattern and marginal tooth form. Leaves of S. hesperia had a length-to-width ratio between 2 and 4.4. Those of S. inquirenda were between 3.6 and 7.5. About 20% of specimens were in the overlapping region between 3.6 and 4.4. The study discusses morphological similarities with modern S. floridana Chapman and S. bonplandiana Kunth, and with a fossil species from the Late Miocene of Austria. Salix hesperia appeared in Late Oligocene or Early Miocene in the Northwest, and S. inquirenda during Middle Miocene. Over time, both species moved southward to California, leaving the West in Late Miocene or Early Pliocene, at about the same time as Taxodium.

Key words: Salix bonplandiana, Salix floridana, Salix hesperia, Salix inquirenda