The mid-Cretaceous dinoflagellate cystAngustidinium acribesDavey & Verdier gen. et comb. nov. displays several morphological features unique among fossil dinoflagellates. On the basis of specimens from northern California, the species is redescribed and is shown to have a paratabulation expressed by the formula 2pr, 5′, 3a, 7″, Xc, 5″′, 2″″; a seven-sided intercalary archeopyle (2a only); an unusually narrow, essentially four-sided first apical paraplate; and typically a small opening or an area of periphragm thickening on each of the antapical paraplates. The distinctive features of the paratabulation are hypothesized to derive from a conventional peridiniacean pattern by subdivision of the mid-dorsal apical paraplate and by ventral shift in the right lateral limit of the first apical paraplate. The pattern is closely similar to the tabulation in living species ofHeterocapsa, notablyH. niei(Loeblich). suggesting affinity with that genus of the Peridiniaceae, rather than toPeridiniumorProtoperidiniumto whcih most fossil peridinoid cysts appear closely allied.