Up to now, the crustacean order Thermosbaenacea contained two genera with only six valid species, five from the panmediterranean region, one from Texas. Two new members of this curious group of “living fossils” have been discovered in the West Indies. The one, from two deep wells not far from the sea coast in Saint Croix (U.S. Virgin Islands), belongs to an undescribed species of the amphiatlantic genus Monodella, and is called M. sanctaecrucis. The other was found in several localities in Curaçao (Netherlands’ Antilles), in coral débris of rubble walls on the shore; it belongs to a new genus, Halosbaena showing several remarkable adaptations, e.g. having a uniramous first pereiopod and reductions in the maxilliped. The presence of very numerous specialized elements on the second maxilla and some other characters relate Halosbaena remotely to a species from Jugoslavia, described as Monodella finki. Several other characters of the latter species justify the erection of a new genus for it, called Limnosbaena. The literature on the possible origin of the Thermosbaenacea is reviewed. Based on the present and other recent discoveries, the origin of the genera Limnosbaena and Monodella (and possibly also of Thermosbaena) at the end of the Tethys period, from marine ancestors, is considered to be the most likely. For the genus Halosbaena a Pleistocene invasion of haline interstitial waters cannot be excluded.