Two of these five genera (the horned _Acrospyris_ and the hornless _Phormospyris_) possess the three typical basal feet of _Cortina_ and _Tripospyris_, and may be derived from these genera by the development of network between the bases of the feet. The three other genera possess numerous feet, and may be derived in the same way from the Polyspyrida; the apical face differs in the three genera, in _Desmospyris_ it is hornless, in _Patagospyris_ it bears an apical horn, and in Rhodospyris three horns (an apical and two lateral or frontal). The cephalis in the Phormospyrida exhibits the same typical structure, which we have described above, of all #Spyroidea# (compare, p. 1017). The thorax or the second joint of the shell arises by reticular connection of the bases of the basal feet, and represents a cylindrical or truncate-conical joint in the polypodal forms, or a three-sided joint in the tripodal forms. The terminal aperture or the mouth of the thorax is constantly open, and surrounded by the free ends of the feet. {1085}_Synopsis of the Genera of Phormospyrida._ I. Subfamily Acrospyrida. } } One apical horn, 474. _Acrospyris_. Three basal cortinar feet } (one odd caudal and two } No apical horn, 475. _Phormospyris_. paired pectoral). } II. Subfamily Rhodospyrida. { One apical horn, 476. _Patagospyris_. { Numerous basal feet in a { Three apical horns, 477. _Rhodospyris_. corona (nine to twelve or { ore.) { No apical horn, 478. _Desmospyris_. Subfamily 1. ACROSPYRIDA, Haeckel, 1881, Prodromus, p. 441 (_sensu emendato_). _Definition._--#Phormospyrida# with three basal feet (an odd caudal and two paired pectoral). Genus 474. _Acrospyris_,[92] Haeckel, 1881, Prodromus, p. 441. _Definition._--#Phormospyrida# with three basal feet and one apical horn. The genus _Acrospyris_ is probably the oldest and most primitive among the Phormospyrida, perhaps the original ancestral form of this family. It has been derived from _Tripospyris_ by the development of lattice-work between the three basal feet, forming a second joint or thorax. It has also therefore a close relation to the simplest forms of the triradiate Dicyrtida: _Dictyophimus_, _Clathrocanium_, &c. It differs from these in the distinct sagittal constriction of the cephalis, which divides it into two lateral loculi. 1. _Acrospyris clathrocanium_, n. sp. (Pl. 95, fig. 17).