PROC. BIOL. SOC. WASH. 97(1), 1984, pp. 142-147 TWO NEW SPECIES OF TUBIFICIDAE (OLIGOCHAETA) FROM THE NORTHERN TERRITORY OF AUSTRALIA Ralph O. Brinkhurst Abstract.— Antipodrilus magelensis, new species, differs from the two other An-tipodrilus species with bifid setae in that the penes have thickened internal base-ment membranes which look like penis sheaths in whole mounted specimens. Rhyacodrilus billabongus, new species, has bifid setae and muscular atria without prostates, as does R. fultoni, but the penial setae and atria differ from those of the latter. This species is discussed in relation to two recently described monotypic marine littoral genera ( Vadicola, Aimidnlus) which have no external prostates on the atria. Magela Creek, Northern Territory, Australia, is a branch of the East Alligator River about 250 km east of Darwin. A series of studies of this system has been made prior to the start of uranium mining in the Ranger deposit, which lies within the watershed. The macroinvertebrate fauna was described by Marchant (1982) but the Oligochaeta have been unidentified until now. Material sent to me by P. M. Outridge from this system consisted of specimens collected in four billabongs (Nankeen, Mudginberri, Georgetown, and Bowerbird) at various dates in 1981-1982. The locations of the billabongs are indicated in Marchant (1982) and are shown on Australian Topographic 1:100,000 series maps 5473 (East Alligator), 5572 (Howship) and 5472 (Cahill). Three genera of the family Naididae and two genera of the family Tubificidae were identified, the tubificid genera being rep-resented by two species new to science. Methods and Materials All of the material was preserved in alcohol. Specimens were either mounted unstained in polyvinyl lactophenol (Gurr) or were stained in borax carmine, dehydrated, and mounted in Canada balsam whole or after dissection in the final mounting medium. Antipodrilus magelensis, new species Figs. 1.2 Holotype.— USNM 80692, a slide-mounted, dissected specimen. Type-locality. — Mudginberri Billabong, Magela Creek, Northern Territory, Australia, mixed sand and silt, 4 May 1981. Paratypes. — USNM 80693-97, 80700, 2 dissected specimens on 2 slides, 6 whole mounted specimens on 3 slides, 19 specimens in fluid. Brinkhurst collection, 15 mature and 39 immature specimens, whole mounted or dissected, on 28 slides. Paratypes from Mudginberri, Georgetown and Bowerbird billabongs, only im-mature specimens from Nankeen Billabong.