The generic diagnosis of Nagmia Nagaty, 1930, is emended to include species having some vitelline follicles extending into the extracecal space. Two new species belonging in the genus are described from the Gulf of California: Nagmia rodmani n. sp. from the body cavity of the giant electric ray, Narcine entemedor Jordan and Starks, 1895, and Nagmia cisloi n. sp. from the body cavity of the smoothtail mobula, Mobula thurstoni (Lloyd, 1908). Both species are distinguished from each other and the 13 other known congeners by the number of testicular follicles, ratio of widths of oral sucker to ventral sucker, and position of the vitelline follicles relative to the ceca. Nagmia rodmani is distinct based on the combined features: 12–17 follicles per testis, sucker width ratio of 1:1.7–2.0, vitelline follicles extending into the extracecal space, and the excretory vesicle being I-shaped and lacking perpendicular lateral branches along the main stem. Nagmia cisloi is distinct based on the combined features: 23–26 follicles per testis, body measuring 13.4 mm long by 14.6 mm wide, eggs measuring 47–48 μm long by 43–47 μm wide, and the excretory vesicle is I-shaped and has lateral perpendicular branches along the main stem. Supplemental data are provided for the anaporrhutine species Probolitrema richiardii (López, 1888) collected from a variety of rays in the Gulf of California. Our specimens are indistinguishable from those described as Probolitrema californiense Stunkard, 1935, and Probolitrema mexicana Markell (1956), both of which previously had been synonymized under P. richiardii. Checklists of parasites reported from N. entemedor and M. thurstoni are provided.