Bull. Calif. Acad. Sci. 1: 181. 1885. 1885
Herbs, annual, terrestrial. Stems erect, terete, prominently glandular-villous. Leaves mostly cauline, opposite; petiole present or absent; blade not fleshy, not leathery, margins entire, venation acrodromous. Inflorescences axillary, flowers paired at nodes, becoming racemelike when nodes condensed; bracts absent. Pedicels present, ± equal to or slightly longer than calyces; bracteoles absent. Flowers mostly erect, not strongly reflexed and appressed in fruit; sepals 5, calyx bilaterally symmetric, tubular, lobes triangular-lanceolate to deltate-lanceolate, midvein low-rounded, not wing-angled; petals marcescent to fugacious, 5, corolla yellow with 2 purple spots on abaxial lip, bilaterally symmetric, weakly to strongly bilabiate, narrowly tubular-funnelform, abaxial lobes 3, adaxial 2; stamens (2 or)4, didynamous, filaments glabrous; ovary 2-locular, placentation parietal; stigma bilamellate. Fruits capsules, apex attenuate, walls densely pustulate-glandular, dehiscence loculicidal along distal 1/3–1/2 of both sutures. Seeds 75–100, brown, narrowly ellipsoid, flattened, wings absent.
w United States, nw Mexico.
Mimetanthe has been recognized by some authors (A. L. Grant 1924; R. F. Hoover 1970; N. H. Holmgren 1984) but not others (F. W. Pennell 1951; P. A. Munz 1959; D. M. Thompson 1993, 2005). Bentham originally described the species in Herpestis; Herpestis is now regarded as a synonym of Bacopa (Plantaginaceae).
The ovary and fruit morphology of Mimetanthe pilosa (apically attenuate fruits without prismatic or angled walls, parietal placentation) are synapomorphic with Diplacus, and M. pilosa could justifiably be included as a basal element, sister to the rest of Diplacus. Fusion of parietal placentae, glandular seeds, and glandular fruit walls in M. pilosa are specializations (C. L. Argue 1980, 1986), and pollen and floral morphology have been noted as distinctions by previous botanists. In treating M. pilosa as the monotypic Mimulus sect. Mimuloides, A. Gray et al. (1886, vol. 2) emphasized the parietal placentation and the deeply divided, essentially unangled calyx.