Tetramerium (Acanthaceae) in the Depresión del Balsas Biogeographic Province of Mexico, two new species and a new combination

A taxonomic account of Tetramerium in the Depresión del Balsas Biogeographic Province of western and central Mexico reveals 14 species there, with 2 of them, T. cruzii and T. michoacanum, described herein. A new combination, T. barlerioides, is proposed for the species previously known as T. ochoterenae. This region of Mexico appears to be the center of both species richness and morphological diversity for this widely distributed American genus. A key to the 14 species is followed by geographic distribution ranges, ecological data within the region, and miscellaneous notes for each one that updates information from the last monograph of the genus. A verified voucher specimen is provided to document species occurrences in each state (or portion of a state) in the Depresión del Balsas. Occurrences of 2 species, T. glandulosum and T. tenuissimum, are newly reported from the Estado de México. A map of the region and floral photographs of 12 of the species are provided.