The range of Malacothamnus hallii is adjacent to M. arcuatus s.s with morphological intermediates where their ranges meet. Phylogenetic analyses place both taxa in the same clade with some analyses indicating some divergence. While future research may show that M. arcuatus and M. hallii should be treated as two species, the 2023 treatment treats M. hallii as M. arcuatus var. elmeri, the elmeri part having taxonomic priority over hallii when treated as a variety. Placing both taxa in the same species allows intermediates to be classified to the species rank.

The suggested common name in the 2023 treatment for M. arcuatus at the species rank (s.l.) is bewildering bushmallow, which alludes to the taxonomic problems from the 2012 Jepson treatment where the author identified specimens of M. arcuatus s.s. as an amazingly large number of other species, presumably as M. arcuatus confounded her analyses. The common names of each variety adds their geographic placement relative to each other. M. arcuatus var. arcuatus is western bewildering bushmallow. M. arcuatus var. elmeri is eastern bewildering bushmallow.

For more information see the new treatment here:
https://doi.org/10.6084/m9.figshare.23937066