Oncomegas Dollfus, 1929 is completely defined for the first time based upon a redescription of the type-species, O. wageneri (Linton, 1890) from Dasyatis centroura (Mitchill), and the description of two new species, O. paulinae from Urolophus halleri Cooper and O. australiensis from Aetobatus narinari (Euphrasen). The three species of Oncomegas are differentiated from one another by scolex ratio, basal and metabasal armatures, testis number and arrangement, number of vaginal sphincters, form of gravid uterus, position of genital pores, and form of the osmoregulatory canals.
Diagnostic characters of Oncomegas are: acraspedote scolex, two bothridia, asymmetrical tentacular basal swelling, basal armature with macrohook, typical heteroacanthous metabasal armature, prebulbar organs, elongate bulbs, retractor muscle origin at base of bulb, external seminal vesicle present, testes in two longitudinal rows or bands, seminal receptacle, lack of a postovarian testicular space, linear gravid uterus with lateral diverticula, and osmoregulatory ducts dorsal to cirrus-sac.
The combination of heteroacanthous typical tentacle armature, two bothridia lacking ciliated pits, presence of prebulbar organs, retractor muscles originating at base of bulbs and posterior ovary warrant the placement of Oncomegas in the Eutetrarhynchidae. Oncomegas is most similar to Eutetrarhynchus Pintner, 1913 in arrangement of hooks of principal rows in opposing half-spirals which abut in ‘V’-formations on both internal and external faces, presence of a basal armature and swelling, presence of an external seminal vesicle, testes confined to the intervascular medullary zone, posterior ovary and circumcortical vitelline field. Oncomegas differs from all eutetrarhynchid genera in the possession of a single macrohook in the basal armature and all except Eutetrarhynchus in the lack of a wide space between hook files 1 and (1′) of the principal rows.