Revision of Parancylodiscoides Caballero y C. & Bravo-Hollis, 1961 (Monogenoidea: Dactylogyridae), with a redescription of P. longiphallus (MacCallum, 1915) from the Atlantic spadefish Chaetodipterus faber (Broussonet) (Acanthuroidei: Ephippidae) in the Gulf of Mexico.

The generic diagnosis of Parancylodiscoides Caballero y C. & Bravo-Hollis, 1961 (Monogenoidea: Dactylogyridae), with three valid species, was amended to include dactylogyrids having: a haptor with dorsal and ventral anchor/bar complexes, 14 hooks (seven pairs) and four reservoirs (two pairs); a dorsal bar with bifurcate ends; hooks with protruding, blunt and slightly depressed thumbs and undilated shanks; a dextroventral vaginal aperture leading to an elongate and oblique vaginal vestibule; a germarium dextral to the testis; a vas deferens looping the left intestinal caecum; a copulatory complex lacking an accessory piece; and two intestinal caeca lacking diverticula and united posterior to the gonads. Parancylodiscoides is most similar to Sundatrema Lim & Gibson, 2009, from which it differs only by the position of the vaginal aperture (sinistroventral in Sundatrema spp.) and by lacking a sucker-like genital pore. New information concerning the number and distribution of haptoral hooks, the relative positions of the gonads and the course of the vas deferens was provided for the type-species, P. chaetodipteri Caballero y C. & Bravo-Hollis, 1961. Parancylodiscoides longiphallus (MacCallum, 1915) Lim & Gibson, 2009 was redescribed based on specimens collected from the Atlantic spadefish Chaetodipterus faber from the Gulf of Mexico off Mississippi and Guaratuba Bay, Paraná, Brazil; its occurrence on the Atlantic spadefish from the Gulf of Mexico, Guaratuba Bay and off Puerto Rico represented new geographical records for this helminth. Parancylodiscoides caballerobravorum Cezar, Luque & Amato, 1999 was considered a junior subjective synonym of P. longiphallus. The monotypic Isohaliotrema Young, 1968 was placed in synonymy with Parancylodiscoides and its type-species, I. platacis Young, 1968, transferred to Parancylodiscoides as Parancylodiscoides platacis (Young, 1968) n. comb.