SYNOPSIS.: Hysterocineta eiseniae, from the oligochaete Ei‐senia lonnbergi, and Ptychostomum campelomae sp. nov., from the fresh‐water prosobranch snail Campeloma geniculum, have been studied primarily with a view to arriving at a better understanding; of the adoral and buccal ciliature of hystero‐cinetid ciliates. In both of these species (as well as in representatives of two undescribed genera), the arrangement of adoral and buccal ciliary organelles is fundamentally the same. There are three adoral rows of cilia extending transversely across the posterior end of the body, toward the buccal cavity. The row nearest the superior surface consists of cilia which are fused into a hyaline and relatively inactive membrane. This row enters the buccal cavity on its left side and extends inward to the cytostome. The other two rows, inferior to the membrane, are close together and consist of active and coherent cilia which collectively form a brush‐like complex. Oi these rows, the one nearer to the membrane enters the buccal cavity on its right side. The other row ends at the right side of the buccal overture, and near the point where it terminates, but on the opposite side of the continuous row, a similar series of cilia originates and closely parallels the continuous row to the cytostome.The diagnosis of the family Hysterocinetidae is emended on the basis of this characteristic arrangement of the adoral and buccal ciliature. The hysterocinetids appear to be rather closely related to ciliates of the family Hemispeiridae, and are therefore tentatively referred to the order Thigmotrichida.