Abstract Nematosphaeropsis balcombiana Deflandre & Cookson 1955 is the genotype of Nematosphaeropsis, but should be restricted to the type material until its morphology can be elucidated by restudy. The circumscription of the genus Nematosphaeropsis is emended, noting in particular the presence of pairs of penitabular trabeculae that join adjacent process terminations. The synonymy of N. balcombiana with “N. labyrinthea”; (Ostenfeld) Reid 1974 is rejected because the dinocysts in these species differ significantly in the type of processes and trabeculae that each exhibits and "N. labyrinthea”; is not considered a valid species of dinoflagellate cyst. N lemniscata Bujak 1984 includes in its circumscription morphologies identical to those specimens attributed to "N. labyrinthea”; by Reid (1974). Two new species of Nematosphaeropsis, N. lativittatus sp. nov. and N. rigida sp. nov., are described from Neogene oceanic deposits from the Gulf of Mexico. Species of Nematosphaeropsis can be differentiated on the basis of gross morphologic features, without having to delineate paratabulation. N. balcombiana has broad, proximally branched processes and thin, delicate trabeculae. Paratabulation is clearly expressed by parasutural membranes that join process bases proximally. N. densiradiata (Cookson & Eisenack) Stover & Evitt 1978 is larger than most species of Nematosphaeropsis and similar to N. lemniscata in structure. N. densiradiata is, however, too poorly known to assess completely. N. downii Brown 1986 contains some of the largest specimens of Nematosphaeropsis and bears pairs of trabeculae fused to each other by thin membranes. N. elegantulum (Drugg) Stover & Williams 1987 is unusual in reportedly having process tips that have 4 or 5 distal branches that may themselves branch, and occasional trabecular beads and barbs. N. lativittatus sp. nov. is small, and bears open processes that are broadly flared distally and that are joined distally by pairs of broad, ribbon‐like penitabular trabeculae that may be more or less fused to each other. N. lemniscata bears long, thin, hollow cylindrical processes joined distally by pairs of uniformly narrow, ribbon‐like penitabular trabeculae. Paratabulation is variably developed on the trabecular shell, but is rarely evident on the central body, and then only partially. N.? philippotii (Deflandre) De Coninck 1969 is too poorly known to assess in detail. N. pustulosa (Morgenroth) Stover & Evitt 1978 is characterized by hollow protrusions or outgrowths of the central body and the very large size of the cysts. N. quinquetrum (Duxbury) Stover & Williams 1987 is distinguished from other species of Nematosphaeropsis by its pentagonal central body. The thin, solid pairs of trabeculae of this species are similar to those of N. rigida sp. nov. N. rigida sp. nov. bears stiff, solid processes that are round in cross‐section and expanded distally and proximally. process terminations are joined by solid trabeculae that are round in cross‐section. N. scala Duxbury 1977 is distinguished by widely flared, distally fenestrate processes and thin trabeculae discontinuously joined to each other by cross strands. N. singularis Davey 1979 bears variably shaped gonal and intergonal processes joined by weak trabeculae. Unipontidinium gen. nov. includes dinocysts with single parasutural trabeculae that join gonal, or gonal and intergonal, processes distally. Proximal parasutural ridges may or may not join process bases. Two new combinations (U. aquaeductum comb. nov. and U. grande comb, nov.) are proposed.