The genus Scalithrium is one of the most poorly known genera of rhinebothriidean tapeworms. It currently houses 10 morphologically heterogeneous species hosted by stingrays and guitarfishes. This study aimed to expand our understanding of this genus. Two new species, Scalithrium healyae n. sp. and Scalithrium johnvolini n. sp., are described from the giant guitarfish, Glaucostegus typus, off Australia and the lesser guitarfish, Acroteriobatus annulatus, off South Africa, respectively. The microtriches of members of this genus are characterized using scanning electron microscopy for the first time. Existing generic membership is critically assessed in the context of the morphology of the type species of the genus, Scalithrium minutum. As a result, Scalithrium bilobatum, Scalithrium smitii, and Scalithrium trygonis are considered species inquirenda owing to the limited amount of available information and material of both species. Scalithrium palombii and Scalithrium rankini are considered incertae sedis given the numerous morphological differences that exist between these species and the type and six other species in the genus. These differences include a greater length, more proglottids, and a much greater number of testes, the distribution of which extends to the ovary, rather than being restricted to the region anterior to the genital pore. The diagnosis of the genus is revised to accommodate these modifications in membership. We anticipate that the more cohesive concept of Scalithrium developed here will help advance future work circumscribing several of the other problematic genera of rhinebothriideans.