Synonyms and other taxonomic changes
These former subgenera are now lumped with subgenus Nomada (sensu stricto) in a single taxon (now a species group as formal subgenera are no longer recognized in Nomada).
This taxon was recognized by Alexander (and later Michener and others) as a formal species group (assemblage of related species) corresponding to multiple former subgenera (e.g., subgenera sensu Mitchell, 1962) including Gnathias, Heminomada, Nomada, Phor and others.
Whereas other species groups of Alexander are well defined and monophyletic, the ruficornis species group is relatively poorly defined and may prove to be non-monophyletic. However, much of the troublesome diversity is in the Old World so not a problem for BugGuide. The problem of delimiting an appropriate ruficornis-group (=Nomada sensu stricto) taxon is one reason why Alexander did not propose a subgeneric system for this genus.
--J.S. Ascher, 13.iii.2008