Of the original taxa referred by Planchon to Arachnothrix, three (A. bud-dleioides, A. elongata, and A. leucophylla) are of Mexican origin, one (A. laniflora) of Central American origin, and two (A. discolor and A. reflexa) of South American origin. The South American taxa placed in Arachnothrix have both reflexed or spreading calyx-lobes. Most of the species of Rondeletia subsequently described from South America have been found to fall within the generic limits now proposed for Arachnothrix. The characters distinguishing this genus from Rondeletia were presented in the introductory discussion under Rondeletia and will not be repeated here. The following key encompasses the known species of Arachnothrix from South America and Trinidad.