The greatest variety of forms is produced in the Zygospyrida by the different number, disposition, and shape of the feet, or the basal apophyses arising from the base of the cephalis. Regarding these remarkable differences, we may distinguish eight subfamilies. The common ancestral group of all are probably the Tripospyrida, which possess the three typical basal feet of Cortina and Cortiniscus, an odd caudal and two paired pectoral feet (Pl. [84]). From these may be derived the Dipospyrida (Pl. [85]) by loss of the caudal foot, the Tetraspyrida by development of an odd sternal foot (Pl. [53], figs. 7, 8, 19), and the Hexaspyrida by development of three secondary interradial between the three primary perradial feet. The Pentaspyrida may have been derived from the latter by loss of the sternal foot (Pl. [95], figs. 9-11), and the Therospyrida by loss of the two sagittal feet (Pl. [89], figs. 5, 6). The Polyspyrida bear numerous (seven to twelve or more, often twenty to thirty) basal feet, forming a corona around the cortinar plate (Pl. [87]); they may have been derived from the Tripospyrida or Hexaspyrida by further multiplication of the feet. Finally, the Circospyrida exhibit no feet at all (Pl. [89], figs. 7-12); they may be derived either from one of the preceding groups by complete reduction and loss of the basal apophyses, or directly from those Semantida which have no feet (Semantis, Semantrum, Clathrocircus, &c.).
Synopsis of the Genera of Zygospyrida.
I. Subfamily Tripospyrida. Three basal feet. (One odd caudal and two paired pectoral.) |  | Apex of the shell with one horn, | 441. Tripospyris. | ||||
| Apex of the shell with three horns, | 442. Triceraspyris. | ||||||
| Apex of the shell without horns, | 443. Tristylospyris. | ||||||
| Apex with two large coryphal holes, right and left, | 444. Cephalospyris. | ||||||
II. Subfamily Dipospyrida. Two basal lateral feet. (Originally the two pectoral feet.) |  | Two feet free, not grown together. |  | Feet simple, not branched, without lateral spines. |  | One horn, | 445. Dipospyris. |
| No horn, | 446. Brachiospyris. | ||||||
| Feet branched like a tree. | | One horn, | 447. Dendrospyris. | ||||
| Feet with a series of lateral spines. | | One horn, | 448. Dorcadospyris. | ||||
| Two feet grown together and uniting to form a ring. Apex with a horn. |  | Feet simple, | 449. Gamospyris. | ||||
| Feet with a series of lateral spines, | 450. Stephanospyris. | ||||||
III. Subfamily Tetraspyrida. Two sagittal and two lateral feet. | | Feet simple, not branched. | | Apex with one horn, | 451. Tetraspyris. | ||
| Apex without a horn, | 452. Tessarospyris. | ||||||
IV. Subfamily Hexaspyrida. Six basal feet. |  | Two sagittal, two pectoral, and two tergal feet. |  | Apex with one horn, | 453. Hexaspyris. | ||
| Apex with three horns, | 454. Liriospyris. | ||||||
| Apex without a horn, | 455. Cantharospyris. | ||||||
V. Subfamily Pentaspyrida. Five basal feet. | | One caudal, two pectoral, and two tergal feet. | | Apex with one horn, | 456. Clathrospyris. | ||
| Apex with three horns, | 457. Ægospyris. | ||||||
| Apex without a horn, | 458. Pentaspyris. | ||||||
VI. Subfamily Therospyrida. Four paired lateral feet. (Two pectoral and two tergal.) | | Apex with one horn, | 459. Zygospyris. | ||||
| Apex with three horns (one apical and two frontal), | 460. Elaphospyris. | ||||||
| Apex with two lateral or frontal horns, | 461. Taurospyris. | ||||||
| Apex without a horn, | 462. Therospyris. | ||||||
VII. Subfamily Polyspyrida. Numerous (seven to twelve or more) basal feet. | | Apex with one horn, | 463. Petalospyris. | ||||
| Apex with three horns (one apical and two frontal), | 464. Anthospyris. | ||||||
| Apex with numerous horns, | 465. Ceratospyris. | ||||||
| Apex without a horn, | 466. Gorgospyris. | ||||||
VIII. Subfamily Circospyrida. No basal feet. | | Apex with one horn, | 467. Circospyris. | ||||
| Apex without a horn, | 468. Dictyospyris. | ||||||
I. Subfamily Tripospyrida. Three basal feet. (One odd caudal and two paired pectoral.) | ||||||||
| Apex of the shell with one horn, | ||||||||
| 441. Tripospyris. | ||||||||
| Apex of the shell with three horns, | ||||||||
| 442. Triceraspyris. | ||||||||
| Apex of the shell without horns, | ||||||||
| 443. Tristylospyris | ||||||||
| Apex with two large coryphal holes, right and left, | ||||||||
| 444. Cephalospyris. | ||||||||
II. Subfamily Dipospyrida. Two basal lateral feet. (Originally the two pectoral feet.) | ||||||||
| Two feet free, not grown together. | ||||||||
| Feet simple, not branched, without lateral spines. | ||||||||
| One horn, | ||||||||
| 445. Dipospyris. | ||||||||
| No horn, | ||||||||
| 446. Brachiospyris. | ||||||||
| Feet branched like a tree. | ||||||||
| One horn, | ||||||||
| 447. Dendrospyris. | ||||||||
| Feet with a series of lateral spines. | ||||||||
| One horn, | ||||||||
| 448. Dorcadospyris. | ||||||||
| Two feet grown together and uniting to form a ring. Apex with a horn. | ||||||||
| Feet simple, | ||||||||
| 449. Gamospyris. | ||||||||
| Feet with a series of lateral spines, | ||||||||
| 450. Stephanospyris. | ||||||||
III. Subfamily Tetraspyrida. Two sagittal and two lateral feet. | ||||||||
| Feet simple, not branched. | ||||||||
| Apex with one horn, | ||||||||
| 451. Tetraspyris. | ||||||||
| Apex without a horn, | ||||||||
| 452. Tessarospyris. | ||||||||
IV. Subfamily Hexaspyrida. Six basal feet. | ||||||||
| Two sagittal, two pectoral, and two tergal feet. | ||||||||
| Apex with one horn, | ||||||||
| 453. Hexaspyris. | ||||||||
| Apex with three horns, | ||||||||
| 454. Liriospyris. | ||||||||
| Apex without a horn, | ||||||||
| 455. Cantharospyris. | ||||||||
V. Subfamily Pentaspyrida. Five basal feet. | ||||||||
| One caudal, two pectoral, and two tergal feet. | ||||||||
| Apex with one horn, | ||||||||
| 456. Clathrospyris. | ||||||||
| Apex with three horns, | ||||||||
| 457. Ægospyris. | ||||||||
| Apex without a horn, | ||||||||
| 458. Pentaspyris. | ||||||||
VI. Subfamily Therospyrida. Four paired lateral feet. (Two pectoral and two tergal.) | ||||||||
| Apex with one horn, | ||||||||
| 459. Zygospyris. | ||||||||
| Apex with three horns (one apical and two frontal), | ||||||||
| 460. Elaphospyris. | ||||||||
| Apex with two lateral or frontal horns, | ||||||||
| 461. Taurospyris. | ||||||||
| Apex without a horn, | ||||||||
| 462. Therospyris. | ||||||||
VII. Subfamily Polyspyrida. Numerous (seven to twelve or more) basal feet. | ||||||||
| Apex with one horn, | ||||||||
| 463. Petalospyris. | ||||||||
| Apex with three horns (one apical and two frontal), | ||||||||
| 464. Anthospyris. | ||||||||
| Apex with numerous horns, | ||||||||
| 465. Ceratospyris. | ||||||||
| Apex without a horn, | ||||||||
| 466. Gorgospyris. | ||||||||
VIII. Subfamily Circospyrida. No basal feet. | ||||||||
| Apex with one horn, | ||||||||
| 467. Circospyris. | ||||||||
| Apex without a horn, | ||||||||
| 468. Dictyospyris. | ||||||||
Subfamily 1. Tripospyrida, Haeckel, 1881, Prodromus, p. 441.
Definition.—Zygospyrida tripoda, with three descending basal feet (the same as in Cortina, an odd posterior or caudal foot and two paired anterior or pectoral feet).
Genus 441. Tripospyris,[[59]] Haeckel, 1881, Prodromus, p. 441.
Definition.—Zygospyrida with three basal feet and one apical horn.
The genus Tripospyris (Pl. [84], figs. 1-7) is in the large group of Spyroidea probably the most original and typical form, from which, as a common ancestral form, all other genera of this polymorphous suborder may be derived. The shell is usually shaped like a nut or thorax, and exhibits two paired lateral chambers, which are separated by a distinct sagittal ring and a slighter or deeper sagittal constriction. The dorsal rod of the ring is prolonged at the upper pole into an ascending apical horn, at the lower pole into a descending caudal foot, whilst two paired pectoral or lateral feet arise from the anterior pole of the basal rod. These three divergent basal feet and the apical horn are the same four typical radial spines which we have encountered already in Plagoniscus (Plagonida), in Plectaniscus (Plectanida), in Cortina (Stephanida), and in Cortiniscus (Semantida). Compare above, p. [891]. From all these Plectellaria, Tripospyris differs in the development of a complete bilocular lattice-shell, having the characters of the true Spyroidea. The numerous species of this genus may be placed in different subgenera, according to the different number of collar pores or cortinar pores in the basal plate (two, three, four, six, or more). These differences are very important, however difficult to make out; the following system therefore is a provisional one, and requires further accurate observations.