Mycteronastes Kearn and Beverley-Burton, 1990 (Monogenoidea: Monocotylidae: Merizocotylinae) was resurrected from subjective synonymy with Merizocotyle Cerfontaine, 1894, and its diagnosis was emended to include monocotylids with a haptor lacking a central loculus and having 5 peripheral (2 bilateral pairs and an unpaired anteromedial loculus), 1 interhamular, and 17 marginal loculi. The 3 species of Mycteronastes accepted herein are parasitic within the olfactory sacs of rays and sawfishes: Mycteronastes icopae (Beverley-Burton and Williams, 1989) Kearn and Beverley-Burton, 1990 (type species) from the giant shovelnose ray, Glaucostegus typus (Anonymous (Bennett)) (Glaucostegidae), in the southwestern Pacific Ocean; Mycteronastes undulatae Kearn and Beverley-Burton, 1990 from the undulate ray, Raja undulata Lacepède (Rajidae), in the northeastern Atlantic Ocean; and Mycteronastes caalusi n. sp. from the smalltooth sawfish, Pristis pectinata Latham (Pristidae), in the Gulf of Mexico. Mycteronastes caalusi is most easily differentiated from its congeners by the combination of having 2 median cephalic papillae, an oval haptor that is wider than the body proper and lacks a deeply scalloped margin, a comparatively large anteromedial peripheral loculus, an unsclerotized male copulatory organ that is wholly anterior to the vaginal pores, a relatively small distal portion of the uterus (ootype chamber) that is mostly anterior to the vaginae, and a delicate uterus. The present study is the first report of a monocotylid from the olfactory sacs of P. pectinata and the first record of a species of Mycteronastes from the Gulf of Mexico. Notes on the taxonomy and systematics of some species assigned to Calicotyle Diesing, 1850 (Monocotylidae: Calicotylinae) are included.