The genus Stuhlmannia Michaelsen 1890 (Pareudrilinae, Eudrilidae) is redefined and a key to species given in the light of the author's examination of material of 24 of the 25 species previously recognized. A discussion of the general morphology of the genus is also included. Twenty-two species are retained and five subspecies of S. variablis are defined. New and suspected synonyms are tabulated. New records are given of S. asymmetrica Michaelsen, 1903a, S. Suctoria (Stephenson), 1932, and S. variabilis Michaelsen, 1890, but these include no significant extension of previously known distribution. A map of the distribution of all known species shows that the genus is limited to tropical Africa, with the great majority of species in east and central Africa within 15 degrees of the equator. S. variabilis, one of the most abundant and widespread limnic magadriles in East Africa, is geographically replaced by closely related species in neighbouring regions, viz. S. asmmetrica in Ethiopia, and S. inermis around Lake Tanganyika. These species, with S. gracilis, which occurs at the western extremity of the range of variabilis, form a remarkably homogeneous and discrete group on the grounds of somatic and genital characteristics. The variabilis group, which has the ovaries enclosed in a complex female apparatus which ensures internal fertilization, would require separation as a distinct genus were it not apparently linked to other members of the genus by intermediate forms. The series which can be distinguished in the progressive complication of the female genital apparatus in the genus endorses union of platydrilus Michaelsen, 1891, with Stuhlmannia by Michaelsen (1936). Diagrams of this series are given and the significane is discussed of the development of a communication between the spermatheca and the oviduct and enclosure of the ovaries in the system thus produced. Evidence is presented for internal fertilization in the variabilis group and in at least some other species of the genus. The hypothesis advanced by Stephenson, (1930a), that the unenclosed condition of the ovaries and absence of a communication between the spermatheca and oviducts are secondary conditions, and his acceptance of a megascolecid origin of the Eudrilidae, are not supported by the present study.