Synopsis of the Families of Spyroidea.
| Shell without thorax; no second shell-joint, developed from the base of the cephalis. |  | Cephalis without galea (or apical cupola), | 1. Zygospyrida. |
| Cephalis with a galea (or apical cupola), | 2. Tholospyrida. | ||
| Shell with a thorax, or a second shell-joint, developed from the base of the cephalis. | | Cephalis without galea (or apical cupola), | 3. Phormospyrida. |
| Cephalis with a galea (or apical cupola), | 4. Androspyrida. |
| Shell without thorax; no second shell-joint, developed from the base of the cephalis. | ||||
| Cephalis without galea (or apical cupola), | ||||
| 1. Zygospyrida. | ||||
| Cephalis with a galea (or apical cupola), | ||||
| 2. Tholospyrida. | ||||
| Shell with a thorax, or a second shell-joint, developed from the base of the cephalis. | ||||
| Cephalis without galea (or apical cupola), | ||||
| 3. Phormospyrida. | ||||
| Cephalis with a galea (or apical cupola), | ||||
| 4. Androspyrida. | ||||
Family LII. Zygospyrida, n. fam. (Pls. [84]-[87]).
Definition.—Spyroidea without galea and thorax; the shell consisting of the bilocular cephalis only and its apophyses.
The family Zygospyrida is by far the richest among the four families of Spyroidea, the number of genera in the whole suborder amounting to forty-five, in the former to twenty-eight; and the number of species in the latter to two hundred and thirty-seven, in the former to one hundred and seventy-two. The number of individuals also found in many species of Zygospyrida is far greater than in any species of the three other families. The shell of the Zygospyrida is represented by the bilocular cephalis only, and never develops a galea (as in the Tholospyrida) nor a thorax (as in the Phormospyrida and Androspyrida). The three latter families have therefore been derived from the former as their common ancestral group.
The Zygospyrida are very similar and nearly related to the Monocyrtida, and in both groups the cephalis alone represents the whole shell. Therefore in 1882, Bütschli, in the paper mentioned above (p. [1016]), maintained the opinion, that these two groups were identical. But there is this important difference between them, that in the Zygospyrida (as in all Spyroidea) the cephalis is bilocular, with a sagittal constriction, separating the right and left chambers. In the Monocyrtida, however (as in all Cyrtoidea), the cephalis is unilocular, forming a quite simple chamber without sagittal constriction. Correspondingly the primary sagittal ring in all Zygospyrida is well preserved and usually complete, while in the Monocyrtida it is never complete, and often quite absent. Another difference is indicated by the form of the central capsule, which in the Zygospyrida is usually bilobed, and more developed in the frontal axis, whilst in the Monocyrtida it is commonly ovate, and more developed in the principal axis.
The important questions of the origin and phylogenetical relation of these two similar groups of Nassellaria form a very complicated and difficult problem, and we do not at present possess the means of solving it. It may be that a part of the Monocyrtida has been derived from the Zygospyrida (as Bütschli erroneously supposes for all), but the contrary is also possible. A third possibility is the independent origin of both groups from the Semantida. But we shall see afterwards, in the description of the Monocyrtida, that a great part of this group may with greater probability be derived from the Plectoidea than from the Stephoidea, and that another part of them has probably been derived quite independently from the Nassellida. Regarding the complicated relations of these similar groups, mentioned above (pp. [892]-[894]), it seems hopeless here to discuss further their difficult affinities; but in any case it seems useful or even necessary to separate the Monocyrtida from the Zygospyrida.
The cephalis of the Zygospyrida possesses all the characteristic features which we have described above as belonging to the Spyroidea in general (p. [1017]), and a survey of the Pls. [84]-[87] exhibits the most important modifications of its shape (compare also Pl. [95], figs. 1-13). Therefore we may avoid useless repetitions by referring to the description given above. We will only point out that the bilocular character of the cephalis (in opposition to the simple cephalis of the Monocyrtida) in the majority of Zygospyrida is apparent on the first view, and that it is always demonstrated by an accurate examination of the sagittal ring and its relation to the longitudinal constriction of the cephalis (compare p. [1019]). The basal plate or cortinar plate exhibits usually four typical pores (two smaller jugular, i, and two larger cardinal, k), but this is by no means a general rule, as Bütschli thought; moreover, instead of four cortinar pores, there are frequently found three or six or some other numbers (compare above, p. [1018]).
The top of the cephalis in the majority of Zygospyrida bears an apical horn, as an upper prolongation of the dorsal rod of the sagittal ring; it is often of unusual size and shape (Pl. [85], figs. 5-11; Pl. [95], fig. 8, &c.). In several genera two lateral or frontal horns are developed on both sides of the former (Pl. [84], figs. 9-12; Pl. [86], figs. 5-13). Sometimes these two alone are present, while the original apical horn is lost by reduction (Pl. [95], fig. 12). In other genera the apical face bears no horns; probably they are lost by reduction.