The Podocyrtida represent one of the largest groups of Cyrtoidea, very rich in numerous species, which are partly very common and widely distributed. A large number of living and fossil species has already been described and figured by Ehrenberg, (1872, 1879, loc. cit.). These mainly belong to his genera Podocyrtis, Pterocanium, Rhopalocanium, Lithornithium, &c., and many of them are very characteristic and common forms, in which the triradial structure of the three-jointed shell is visible at first view. We distinguish here twenty genera and one hundred and fifty species. The majority have a shell with a simple wide open mouth (Theopilida, fourteen genera and one hundred and eighteen species), in the minority the mouth is closed by lattice-work (Theoperida, six genera and thirty-two species).
The three joints of the shell have in the majority of Podocyrtida such a proportion that the cephalis is the smallest, the abdomen the largest, and the thorax between them intermediate in size. The cephalis bears almost constantly an apical horn; this is rarely reduced or lost; sometimes two or more horns are developed. The three radial apophyses arise originally from the base of the cephalis, and may from this point run along the shell-wall embedded in the lattice-plate of the thorax and the abdomen. They leave the latter at very different points, and form either lateral wings or terminal feet, sometimes both together. They are either solid or latticed, sometimes also branched, and of very various shape. Often only the three terminal feet surrounding the mouth have remained, whilst the ribs have disappeared.
Many species of this large family are very variable and connected with other different species by numerous transitional forms; the distinction of the genera described is also often very difficult.
Synopsis of the Genera of Podocyrtida.
I. Subfamily Theopilida. (Podocyrtida aperta.) Terminal mouth of the shell a simple wide opening. |  | A. Three free limbs or wings on the thorax (partly also on the abdomen). |  | No free external appendages on the abdomen. |  | Three wings of the thorax solid. |  | Wings arising from the thorax, | 582. Pterocorys. |
| Three ribs enclosed in the wall of the thorax, | 583. Theopilium. | ||||||||
| Wings arising free from the collar stricture, | 584. Corocalyptra. | ||||||||
| Three wings of the thorax latticed. |  | Wings not prolonged into the cephalis, | 585. Dictyoceras. | ||||||
| Wings prolonged into the cephalis, | 586. Pteropilium. | ||||||||
| Free appendages on the abdomen. |  | Three ribs prolonged into three terminal feet. |  | Ribs and feet solid, | 587. Theopodium. | ||||
| Ribs and feet latticed, | 588. Pterocanium. | ||||||||
| Three free wings on the thorax. Numerous terminal feet. |  | Wings and feet solid, | 589. Pterocodon. | ||||||
| Wings and feet latticed, | 590. Dictyocodon. | ||||||||
| B. Free appendages not on the thorax, only on the abdomen. | | Abdomen with three ribs and three feet, | 591. Pleuropodium. | ||||||
| Abdomen without lateral ribs, with three terminal feet. |  | Three feet solid, simple, | 592. Podocyrtis. | ||||||
| Three feet solid, ramified, | 593. Thyrsocyrtis. | ||||||||
| Three feet latticed, | 594. Dictyopodium. | ||||||||
II. Subfamily Theoperida. (Podocyrtida clausa.) Terminal mouth of the shell closed by a lattice-plate. |  | Three lateral wings only on the thorax, |  | Wings solid, | 595. Lithornithium. | ||||
| Wings latticed, | 596. Sethornithium. | ||||||||
| Three lateral wings beginning from the thorax and prolonged into the abdomen, | 597. Theopera. | ||||||||
| Three wings only on the abdomen. |  | Spell spindle-shaped, with three lateral wings on the abdomen. | | No horn on the basal pole, | 598. Rhopalocanium. | ||||
| Basal pole of the shell with a horn, | 599. Rhopalatractus. | ||||||||
| Shell three-sided pyramidal, with three terminal-feet on the basal corners, | 600. Lithochytris. | ||||||||
I. Subfamily Theopilida. (Podocyrtida aperta.) Terminal mouth of the shell a simple wide opening. | ||||||||||
| A. Three free limbs or wings on the thorax (partly also on the abdomen). | ||||||||||
| No free external appendages on the abdomen. | ||||||||||
| Three wings of the thorax solid. | ||||||||||
| Wings arising from the thorax, | ||||||||||
| 582. Pterocorys. | ||||||||||
| Three ribs enclosed in the wall of the thorax, | ||||||||||
| 583. Theopilium. | ||||||||||
| Wings arising free from the collar stricture, | ||||||||||
| 584. Corocalyptra. | ||||||||||
| Three wings of the thorax latticed. | ||||||||||
| Wings not prolonged into the cephalis, | ||||||||||
| 585. Dictyoceras. | ||||||||||
| Wings prolonged into the cephalis, | ||||||||||
| 586. Pteropilium. | ||||||||||
| Free appendages on the abdomen. | ||||||||||
| Three ribs prolonged into three terminal feet. | ||||||||||
| Ribs and feet solid, | ||||||||||
| 587. Theopodium. | ||||||||||
| Ribs and feet latticed, | ||||||||||
| 588. Pterocanium. | ||||||||||
| Three free wings on the thorax. Numerous terminal feet. | ||||||||||
| Wings and feet solid, | ||||||||||
| 589. Pterocodon. | ||||||||||
| Wings and feet latticed, | ||||||||||
| 590. Dictyocodon. | ||||||||||
| B. Free appendages not on the thorax, only on the abdomen. | ||||||||||
| Abdomen with three ribs and three feet, | ||||||||||
| 591. Pleuropodium. | ||||||||||
| Abdomen without lateral ribs, with three terminal feet. | ||||||||||
| Three feet solid, simple, | ||||||||||
| 592. Podocyrtis. | ||||||||||
| Three feet solid, ramified, | ||||||||||
| 593. Thyrsocyrtis. | ||||||||||
| Three feet latticed, | ||||||||||
| 594. Dictyopodium. | ||||||||||
II. Subfamily Theoperida. (Podocyrtida clausa.) Terminal mouth of the shell closed by a lattice-plate. | ||||||||||
| Three lateral wings only on the thorax, | ||||||||||
| Wings solid, | ||||||||||
| 595. Lithornithium. | ||||||||||
| Wings latticed, | ||||||||||
| 596. Sethornithium. | ||||||||||
| Three lateral wings beginning from the thorax and prolonged into the abdomen, | ||||||||||
| 597. Theopera. | ||||||||||
| Three wings only on the abdomen. | ||||||||||
| Spell spindle-shaped, with three lateral wings on the abdomen. | ||||||||||
| No horn on the basal pole, | ||||||||||
| 598. Rhopalocanium. | ||||||||||
| Basal pole of the shell with a horn, | ||||||||||
| 599. Rhopalatractus. | ||||||||||
| Shell three-sided pyramidal, with three terminal-feet on the basal corners, | ||||||||||
| 600. Lithochytris. | ||||||||||
Subfamily 1. Theopilida, Haeckel, Prodromus, p. 435.
Definition.—Podocyrtida with the terminal mouth of the shell open (vel Tricyrtida triradiata aperta).
Genus 582. Pterocorys,[[199]] Haeckel, 1881, Prodromus, p. 435.
Definition.—Theopilida (vel Tricyrtida triradiata aperta) with three simple, free lateral wings arising from the sides of the thorax.