Madagascar has been isolated from other land for about 100 million years. Thus the hydra of the island have been evolving independently of the hydra on the continents during the Tertiary geological period. On the northern continents the species of hydra cluster into four groups. Three species were found in a search of about 100 sites across Madagascar. These species represent two, and perhaps three, of the four groups of species that are found on the northern continents. A green hydra found throughout the island is similar to the European Hydra viridissima. One type of brown hydra, H madagascarensis n.sp., has characters intermediate between those of the vulgaris and braueri groups of continental brown hydra. The third species, unnamed in this paper, belongs to the vulgaris group. One continental group of hydra, the oligactis group, is not represented on Madagascar. The hydra that occur in Madagascar are very similar to some on the continents, indicating that these animals evolve exceedingly slowly.
