The number of genera, however, is comparatively small, and their distinction difficult, since the form of the shell presents no striking differences.
It exhibits all possible passages, from a flatly expanded or nearly discoidal form with a wide open mouth, to a conical or cylindrical, and by constriction of the terminal mouth to an ovate or spindle-shaped form. By complete closure of the mouth the Theocapsida arise. In some genera remarkable traces of the original triradial structure are preserved. The most interesting of these forms is Axocorys, with an internal axial columella, which bears a number of verticils, each with three radial branches.
Synopsis of the Genera of Theocyrtida.
I. Subfamily Theocorida. Terminal mouth of the abdomen a simple wide opening. |  | Abdomen gradually dilated towards the wide open mouth. |  | Abdomen flatly expanded, discoidal. |  | With horn, | 611. Theocalyptra. |
| No horn, | 612. Cecryphalium. | ||||||
| Abdomen conical. | | One horn, | 613. Theoconus. | ||||
| Two or more horns, | 614. Lophoconus. | ||||||
| Abdomen cylindrical, of nearly equal breadth throughout its whole length. Mouth truncate. |  | A single apical horn on the cephalis. | | Thorax and abdomen of equal breadth, | 615. Theocyrtis. | ||
| Thorax much broader than the tubular abdomen, | 616. Theosyringium. | ||||||
| Two horns or a bunch of horns on the cephalis, | 617. Lophocyrtis. | ||||||
| No horn on the cephalis, | 618. Tricolocampe. | ||||||
| Abdomen ovate or inversely conical. Mouth more or less constricted. |  | A single apical horn on the cephalis. | | Shell-cavity without an internal columella, | 619. Theocorys. | ||
| Shell-cavity with an axial columella, | 620. Axocorys. | ||||||
| Two horns or a bunch of horns on the cephalis, | 621. Lophocorys. | ||||||
| No horn on the cephalis, | 622. Theocampe. | ||||||
II. Subfamily Theocapsida. Terminal mouth closed by a lattice-plate. | | No latticed septum between thorax and abdomen. |  | An apical horn, | 623. Theocapsa. | ||
| No horn on the cephalis, | 624. Tricolocapsa. | ||||||
| A complete latticed septum between thorax and abdomen, | 625. Phrenocodon. | ||||||
I. Subfamily Theocorida. Terminal mouth of the abdomen a simple wide opening. | ||||||||
| Abdomen gradually dilated towards the wide open mouth. | ||||||||
| Abdomen flatly expanded, discoidal. | ||||||||
| With horn, | ||||||||
| 611. Theocalyptra. | ||||||||
| No horn, | ||||||||
| 612. Cecryphalium. | ||||||||
| Abdomen conical. | ||||||||
| One horn, | ||||||||
| 613. Theoconus. | ||||||||
| Two or more horns, | ||||||||
| 614. Lophoconus. | ||||||||
| Abdomen cylindrical, of nearly equal breadth throughout its whole length. Mouth truncate. | ||||||||
| A single apical horn on the cephalis. | ||||||||
| Thorax and abdomen of equal breadth, | ||||||||
| 615. Theocyrtis. | ||||||||
| Thorax much broader than the tubular abdomen, | ||||||||
| 616. Theosyringium. | ||||||||
| Two horns or a bunch of horns on the cephalis, | ||||||||
| 617. Lophocyrtis. | ||||||||
| No horn on the cephalis, | ||||||||
| 618. Tricolocampe. | ||||||||
| Abdomen ovate or inversely conical. Mouth more or less constricted. | ||||||||
| A single apical horn on the cephalis. | ||||||||
| Shell-cavity without an internal columella, | ||||||||
| 619. Theocorys. | ||||||||
| Shell-cavity with an axial columella, | ||||||||
| 620. Axocorys. | ||||||||
| Two horns or a bunch of horns on the cephalis, | ||||||||
| 621. Lophocorys. | ||||||||
| No horn on the cephalis, | ||||||||
| 622. Theocampe. | ||||||||
II. Subfamily Theocapsida. Terminal mouth closed by a lattice-plate. | ||||||||
| No latticed septum between thorax and abdomen. | ||||||||
| An apical horn, | ||||||||
| 623. Theocapsa. | ||||||||
| No horn on the cephalis, | ||||||||
| 624. Tricolocapsa. | ||||||||
| A complete latticed septum between thorax and abdomen, | ||||||||
| 625. Phrenocodon. | ||||||||
Subfamily 1. Theocorida, Haeckel, 1881, Prodromus, p. 434.
Definition.—Theocyrtida with the basal mouth of the shell open (vel Tricyrtida eradiata aperta).
Genus 611. Theocalyptra,[[228]] Haeckel, 1881, Prodromus, p. 434.
Definition.—Theocorida (vel Tricyrtida eradiata aperta) with discoidal or flatly expanded abdomen. Cephalis with a single apical horn, or with two divergent horns.
The genus Theocalyptra, and the following closely allied genus Cecryphalium, differ from all the other Theocorida in the flattened form of the cap-shaped or nearly discoidal shell. The abdomen specially is quite flat, often horizontally expanded, and like the marginal brim of a flat hat. They may be derived from the Dicyrtida Sethocephalus by the development of an abdomen, or from Corocalyptra by loss of the collar wings. The network is usually very delicate, as in the latter. The cephalis of Theocalyptra bears one or two horns.