Forticulcita platana sp. n. and Forticulcita apiensis sp. n. are described from Mugil liza Valenciennes in Argentina, and from Mugil cephalus Linnaeus in Salt Springs, Florida, USA, respectively. Supplemental material relating to the hermaphroditic sac of Forticulcita gibsoni Blasco-Costa, Montero, Balbuena, Raga et Kostadinova, 2009 is provided from a specimen isolated from M. cephalus off Crete, Greece. Forticulcita platana can be distinguished from all species of Forticulcita Overstreet, 1982 except F. gibsoni, based on possessing small pads or gland cells along the hermaphroditic duct. It can be differentiated from that species in possessing a hermaphroditic sac that is one and a half to two times longer than wide rather than one that is approximately three times longer than wide, longer eggs (44-52 μm rather than 34-44 μm long) and a shorter post-testicular space (< 45% of the body length). Forticulcita apiensis can be differentiated from the other species of Forticulcita in possessing a testis that is shorter than or equal to the pharynx rather than one that is longer than the pharynx. Xiha gen. n. is erected for Dicrogaster fastigatus Thatcher et Sparks, 1958 as Xiha fastigata (Thatcher et Sparks, 1958) comb. n., and we tentatively consider Dicrogaster fragilis Fernández Bargiela, 1987 to be Xiha fragilis (Fernández Bargiela, 1987) comb. n. The new genus fits within the concept of Forticulcitinae Blasco-Costa, Balbuena, Kostadinova et Olson, 2009 in having a vitellarium comprised of a single elongate to subspherical mass. Xiha can be differentiated from Forticulcita in having spines lining the hermaphroditic duct, or intromittent organ. A Bayesian inference analysis of partial 28S rDNA sequences of the two New World species of Forticulcita, Xiha fastigata and previously published haploporids places Xiha fastigata within the Forticulcitinae and sister to Forticulcita. Amended diagnoses for the subfamily and for Dicrogaster Looss, 1902 are provided.