Abstract
The history of the genus Pachypanchax Myers, 1933 in the literature is reviewed and the utility of the diagnostic characters proposed by various authors is evaluated. On the basis of five synapomorphies, four skeletal and one squamational, six of the seven presently known Malagasy aplocheilids are found to be unambiguously referable to the genus Pachypanchax. The seventh, Poecilia nuchimaculata Guichenot 1866, known only from the unique type specimen, displays several peculiar skeletal and squamational features. Pending the acquisition of additional material, it is tentatively assigned to the genus. Of the six species treated here, Pachypanchax omalonotus (Duméril, 1861) and P. sakaramyi (Holly, 1828) are redescribed from recently collected topotypical material; and the following four are described as new: P. varatraza., P. patriciae, P. sparksorum, and P. arnoulti. Data on life colors, distribution, natural history and conservation status on all six Malagasy Pachypanchax species are presented.

Key words: Aplocheilidae, biogeography, conservation, distribution, arnoulti, Madagascar, natural history, Pachypanchax, nuchimaculatus, omalonotus, patriciae, playfairii, sakaramyi, sparksorum, varatraza

Introduction
Myers (1933) established the genus Pachypanchax for Haplochilus playfairii Günther, 1866, a robust killifish endemic to the granitic Seychelles (Playfair and Günther, 1866). He included two Malagasy species, Poecilia omalonota Duméril, 1861 and Panchax sakaramyi Holly, 1928 in his new genus. This attribution appears to have been made on biogeographic grounds, as Myers clearly indicated that he had not examined specimens of either Malagasy species. Scheel (1968) refined Myer’s definition of the genus with the addition of two additional diagnostic characters. However, the material of P. omalonota available to Scheel consisted of aquarium-reared specimens descended from fish collected from the lower reaches of the westward-flowing Betsiboka River, on the mainland of Madagascar (Arnoult, 1955), whereas the type material of P. omalonotus actually came from the island of Nosy Be, off northwestern Madagascar. Subsequent collecting has demonstrated these two taxa differ sufficiently in coloration and morphology to warrant recognition as separate species. Parenti (1981) subsequently redefined the genus, placing particular emphasis upon several skeletal apomorphies. However, Parenti neither examined topotypical material of P. omalonotus nor specimens of the other two nominal Malagasy species, P. sakaramyi and P. nuchimaculatus (Guichenot, 1866). Her inclusion of the Malagasy species within the genus was thus tentative. Fieldwork in Madagascar undertaken since 1993 has resulted in the collection of larger series of both P. omalonotus and P. sakaramyi, and in the discovery of additional aplocheilid taxa that cannot be referred to previously published nomina...