Sethocorida et Sethocapsida, Haeckel, 1881, Prodromus, pp. 439, 433.
Definition.—Dicyrtida eradiata. (Cyrtoidea with a two-jointed shell, divided by a transverse constriction into cephalis and thorax, without radial apophyses).
The family Sethocyrtida, composed of the Sethocorida and Sethocapsida of my Prodromus, comprises those Cyrtoidea in which the shell is two-jointed and bears no radial apophyses. The two subfamilies differ in the shape of the mouth, which in the Sethocorida is a simple wide opening, and in the Sethocapsida is closed by a lattice-plate. The former are here divided into eight, the latter into three genera.
Numerous living and fossil forms of Sethocyrtida were previously described by Ehrenberg, partly in his genera Dictyocephalus and Lophophæna, partly (intermingled with three-jointed and many-jointed forms) in the genera Eucyrtidium and Lithocampe. A larger number of new forms has been found in the Radiolarian ooze collected by the Challenger, and others fossil in Barbados. Many of these Sethocyrtida are common and widely distributed. When a new third joint is formed on their basal mouth, they pass over into Theocyrtida.
The cephalis is usually subspherical or hemispherical, and much smaller than the thorax, the form of which exhibits all possible intermediate stages between flat, discoidal, conical, cylindrical, and ovate forms. The wide open mouth becomes more and more constricted, and finally closed in the Sethocapsida. The majority of the Sethocyrtida may have been derived either from the Tripocyrtida, or from the Anthocyrtida by reduction and loss of the radial apophyses.
Synopsis of the Genera of Sethocyrtida.
I. Subfamily Sethocorida. Terminal mouth of the thorax a simple wide opening. |  | Thorax conical or campanulate, gradually dilated. |  | Shell simple, without mantle, | 572. Sethoconus. | ||
| Shell double, with an arachnoid mantle, | 573. Periarachnium. | ||||||
| Thorax discoidal, flatly expanded. |  | Cephalis very large, without horn, | 574. Sethocephalus. | ||||
| Thorax cylindrical or ovate, with truncate, constricted or tubular mouth. |  | Cephalis with a single horn. | | Mouth simple, | 575. Sethocyrtis. | ||
| Mouth tubular, | 576. Sethocorys. | ||||||
| Cephalis with two horns or a bunch of horns, | 577. Lophophæna. | ||||||
| Cephalis without horn, | 578. Dictyocephalus. | ||||||
II. Subfamily Sethocapsida. Terminal mouth closed by a lattice-plate. |  | Cephalis free, not hidden in the thorax. | | One horn, | 579. Sethocapsa. | ||
| No horn, | 580. Dicolocapsa. | ||||||
| Cephalis hidden in the upper part of the thorax. | No horn, | 581. Cryptocapsa. | |||||
I. Subfamily Sethocorida. Terminal mouth of the thorax a simple wide opening. | ||||||||
| Thorax conical or campanulate, gradually dilated. | ||||||||
| Shell simple, without mantle, | ||||||||
| 572. Sethoconus. | ||||||||
| Shell double, with an arachnoid mantle, | ||||||||
| 573. Periarachnium. | ||||||||
| Thorax discoidal, flatly expanded. | ||||||||
| Cephalis very large, without horn, | ||||||||
| 574. Sethocephalus. | ||||||||
| Thorax cylindrical or ovate, with truncate, constricted or tubular mouth. | ||||||||
| Cephalis with a single horn. | ||||||||
| Mouth simple, | ||||||||
| 575. Sethocyrtis. | ||||||||
| Mouth tubular, | ||||||||
| 576. Sethocorys. | ||||||||
| Cephalis with two horns or a bunch of horns, | ||||||||
| 577. Lophophæna. | ||||||||
| Cephalis without horn, | ||||||||
| 578. Dictyocephalus. | ||||||||
II. Subfamily Sethocapsida. Terminal mouth closed by a lattice-plate. | ||||||||
| Cephalis free, not hidden in the thorax. | ||||||||
| One horn, | ||||||||
| 579. Sethocapsa. | ||||||||
| No horn, | ||||||||
| 580. Dicolocapsa. | ||||||||
| Cephalis hidden in the upper part of the thorax. | ||||||||
| No horn, | ||||||||
| 581. Cryptocapsa. | ||||||||
Subfamily 1. Sethocorida, Haeckel, Prodromus, p. 430.
Definition.—Sethocyrtida with the basal mouth of the shell open (vel Dicyrtida eradiata aperta).