Millerichthys Costa.

Monotypic (( M. robustus (Miller & Hubbs)) small-sized (30 mm SL), annual genus, that is distributed in southern Mexico. The species is diagnosed by the following apomorphies: reduced mesopterygoid; conical and elongate ventral process of posttemporal; long posterior series of supraorbital neuromasts; absence of transverse dark bar crossing the eye; three transverse yellow stripes on the anal fin of males; and black spots along the caudal-fin base of females (Costa, 1998). Its phylogenetic relationship with other Rivulidae remains unclear. Costa (1998), proposed three synapomorphies to support the sister relationship of Millerichthys Costa with Rachoviini: a dorsally pointed preopercle, reduced neural prezygapophyses of caudal vertebrae, and a reduced dark pigmentation in females. However, Costa (2004) hypothesized the genus as the sister taxon of Cynolebiasinae based on three different synapomorphies: mesopterygoid small, posterior tip not contacting metapterygoid; hypurals completely fused; frontal scales in transverse pattern without distinct central scale.