As part of the Euphorbia Planetary Biodiversity Inventory project (Esser & al., 2009), we are attempting to document and account for all Euphorbia L. names and their phylogenetic placement in the genus. Euphorbia sect. Anisophyllum Roep. (the former genus Chamaesyce Gray) consists mainly of small, prostrate herbs with opposite leaves and tiny flower-like cyathia. It is occasionally confused with taxa in other families that bear a superficial resemblance to it, most notably with certain Lythraceae. Recently, Berry & Phillipson (2011) demonstrated that a species described as Euphorbia benoistii Leandri (Leandri, 1947) was in reality a species of the endemic Madagascan Lythraceae genus Capuronia Lourteig. Here we document a second instance in which both Baillon (1886) and Leandri (1947) were fooled into recognizing a Euphorbia, E. hildebrandtii Baill., that was in reality a member of the Lythraceae. Since this name was published before a corresponding name was available in Lythraceae, it now requires a new name for the species of Nesaea Kunth under which it was published. We establish the new combination below.

Observations. — This is an unusual, but certainly not unprecedented, case of the same collection serving as the type for two species in different families. The specimens that were cited as types of Nesaea pubescens were distributed with labels in Hildebrandt's handwriting identified as “Ammannia,” whereas those cited as types of Euphorbia hildebrandtii were distributed identified as such in Hildebrandt's same handwriting. Since the holotype of Nesaea pubescens was destroyed at B, we chose the specimen at K as the lectotype, since it is the most complete specimen remaining and was also annotated by Koehne himself as N. pubescens. From the type specimens cited above, there were three separate mounted sheets that ended up at both M and P. With the recognition here that the two species were based on the same collection number, and that all of the duplicates in question do indeed correspond to the same species, they all become type material of their respective names.

Nesaea is a mainly African genus of around 55 species, six of which are found on Madagascar, four of them endemic there (Perrier de la Bâthie, 1954, not counting N. crassicaulis (Guill. & Perr.) Koehne, which was erroneously attributed to Madagascar based on a blank-labeled Bojer collection that probably came from Zanzibar). Graham (2007) indicates that Nesaea may be congeneric with Ammannia L., which has a cosmopolitan distribution. Nesaea hildebrandtii is one of the four endemic species on Madagascar and is so far known only from the sandy banks of the Betsiboka River and its tributary the Ikopa River, both in Mahajanga Province.

Additional specimens examined. — Madagascar. Prov. Mahajanga: bords de la Betsiboka, Perrier de la Bâthie 17928 (P [P05084276]); sables de l'Ikopa, Firingalava, VI.1898, Perrier de la Bâthie 747 (P [P05084273 ex Herbier Drake]!); rives et îlots de l'Ikopa en amont de Maevatanana, à Tsarasaotra, 1892, Perrier de la Bâthie 294 (P [P05084272 ex Herbier Drake]!).