But there was no mint-like fragrance to the leaves...and the ''wings'' were quite widely-spreading, while the ''keel'' was not so tightly-closed above as in a typical pea (and, also, the leaves were simple...which is unusual for Fabaceae).

Of course, there are other taxa with (pseudo-)papilionaceous flowers...in particular, I was familiar with Carlowrightia...which have very similar-looking flowers, and also opposite leaves. That hinted at the correct family here, Acanthaceae...and this went to genus Tetramerium in Kearney & Peebles' ''Arizona Flora''. The species name given there was T. hispidum, but the current name is T. nervosum per SEINET and Daniel(2016) from the 'Canotia: Vascular Plants of Arizona' website. The location here is consistent with the distribution indicated on the SEINET map, as well as the more coarse county-level BONAP map for the US portion of the range.

For those who may be interested, here is Bentham's original 1846 description (as determined and transmitted to him by Nees von Esenbeck). This is a very widespread species, its range extending from AZ, NM, and TX to Peru. Daniel(1986) gives a very detailed treatment of the genus.