The definition ofMarcusaedrilusRighi&Kanner, 1979, is modified to includeM. hummelinckiRighi&Kanner, 1979,M. luteolussp.n. (both from the Caribbean), and six new species from the Great Barrier Reef,M. grandiculussp.n.,M. minusculussp.n.,M. tubersp.n.,M. capricornaesp.n.,M. sacculatussp.n., andM. irregularissp.n. The genus is largely distinguished from the closely relatedLimnodriloidesPierantoni, 1903, by its partially ciliated atria and its tripartite spermathecae.