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.). {1024}_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 {Feet simple, }One horn, 445. _Dipospyris_. grown together. { not branched, } { without } { lateral }No horn, 446. _Brachiospyris_. { spines. } { {Feet branched }One horn, 447. _Dendrospyris_. { like a tree. } { {Feet with a } { series of }One horn, 448. _Dorcadospyris_. { lateral } { spines. }