Section III. TRICYRTIDA, Haeckel, 1881, Prodromus, p. 426, 434.
Definition.—Cyrtoidea trithalamia, with three-jointed shell, composed of cephalis, thorax, and abdomen, with two separating transverse constrictions, an upper collar, and a lower lumbar constriction.
Synopsis of the Families and Subfamilies of Tricyrtida.
Family LXV. Podocyrtida. Three radial apophyses. |  | Mouth open, | Theopilida. |
| Mouth closed, | Theoperida. | ||
Family LXVI. Phormocyrtida. Numerous radial apophyses. | | Mouth open, | Theophormida. |
| Mouth closed, | Theophænida. | ||
Family LXVII. Theocyrtida. No radial apophyses. | | Mouth open, | Theocorida. |
| Mouth closed, | Theocapsida. |
Family LXV. Podocyrtida. Three radial apophyses. | ||||
| Mouth open, | ||||
| Theopilida. | ||||
| Mouth closed, | ||||
| Theoperida. | ||||
Family LXVI. Phormocyrtida. Numerous radial apophyses. | ||||
| Mouth open, | ||||
| Theophormida. | ||||
| Mouth closed, | ||||
| Theophænida. | ||||
Family LXVII. Theocyrtida. No radial apophyses. | ||||
| Mouth open, | ||||
| Theocorida. | ||||
| Mouth closed, | ||||
| Theocapsida. | ||||
Family LXV. Podocyrtida, n. fam. (Pl. [67], [68], [71]-[73]).
Theopilida et Theoperida, Haeckel, 1881, Prodromus, p. 435, 436.
Definition.—Tricyrtida triradiata. (Cyrtoidea with a three-jointed shell, divided by two transverse constrictions into cephalis, thorax, and abdomen, and bearing three radial apophyses.)
The family Podocyrtida, composed of the Theopilida and Theoperida of my Prodromus, comprises those Cyrtoidea in which the lattice-shell is three-jointed and triradial, bearing three external apophyses. The two subfamilies differ in the shape of the basal mouth, which in the Theopilida is a simple wide opening, in the Theoperida closed by a lattice-plate. The latter are derived from the former by development of this closing plate. The phylogenetic origin of the Podocyrtida may be found in the Tripocyrtida, from which they are derived by development of an abdomen. But there are also some forms, which may be derived directly from the Plectoidea, and the origin of some other forms seems to be doubtful.