Four basal feet (and { Two sagittal and two an apical horn). { lateral feet, 406. _Stephanium_. Subfamily 1. LITHOCIRCIDA, Haeckel. _Definition_.--#Stephanida# without typical basal feet or cortinar feet. Genus 401. _Archicircus_,[20] n. gen. _Definition_.--#Stephanida# with a simple amphithect or diphragmatic ring, smooth or thorny, without branched spines and basal feet. The genus _Archicircus_ is the most primitive and simplest form of all #Stephoidea#, and probably the common ancestral form, not only of this suborder, but of the greater number of all NASSELLARIA (compare above, p. 893). The skeleton consists only of a quite simple sagittal ring, in which commonly a slight difference of both poles of the main axis (basal and apical pole) is visible, but no difference between the dorsal and the ventral bow of the ring. In my Prodromus (1881, p. 447) the species of _Archicircus_ were disposed partly in the subgenus _Monostephus_, partly in the genus _Lithocircus_. Subgenus 1. _Monostephus_, Haeckel, 1881, Prodromus, p. 447. _Definition._--Ring circular, elliptical, or ovate, without prominent corners. 1. _Archicircus princeps_, n. sp. Gate circular. Ring circular, smooth, its transverse section also circular. The simplest form of all #Stephoidea#. _Dimensions._--Diameter of the gate 0.05 to 0.08; thickness of the ring 0.006 to 0.008. _Habitat._--Central Pacific, Stations 265 to 274, depth 2350 to 2925 fathoms. {941}2. _Archicircus monostephus_, n. sp. (Pl. 83, fig. 17). Gate circular. Ring circular, with three elegantly denticulate edges (one outer median and two lateral edges). Transverse section of the ring triangular. _Dimensions._--Diameter of the gate 0.06 to 0.08; thickness of the ring 0.01 to 0.008. _Habitat._--Central Pacific, Station 274, depth 2750 fathoms. 3. _Archicircus ellipsis_, n. sp. Gate elliptical. Ring elliptical, on the inner margin smooth, on the outer margin with fifteen to twenty equal, short, simple, or slightly forked thorns. Transverse section ovate. _Dimensions._--Diameter of the gate 0.05 to 0.09; thickness of the ring 0.01 to 0.014. _Habitat._--Western Tropical Pacific, Station 225, depth 4475 fathoms; also fossil in Barbados. 4. _Archicircus ovalis_, n. sp. Gate ovate. Ring elliptical, smooth, with three prominent, slightly distorted edges, without thorns; in the transverse section triangular. _Dimensions._--Diameter of the gate 0.08 to 0.12; thickness of the ring 0.006 to 0.009. _Habitat._--Tropical Atlantic, Station 347, depth 2250 fathoms. 5. _Archicircus monopylus_, n. sp. Gate ovate. Ring ovate, thorny, with three distorted edges, in the transverse section triangular; surface covered with numerous simple small thorns arising from the three edges. _Dimensions._--Diameter of the gate 0.1 to 0.15; thickness of the ring 0.01 to 0.012. _Habitat._--Indian Ocean (Madagascar), Rabbe, surface. Subgenus 2. _Archistephus_, Haeckel.