ment of this ring and coincident sagittal constriction of the cephalis the order Spyroidea may be derived from the Cyrtoidea. On the other hand, the Plectellaria, which possess no cephalis, and indeed no complete lattice-shell whatever, may be derived from the Monocyrtida by the assumption of a degeneration of this structure; the sagittal ring having been preserved in the Stephoidea, and the tripod of the Tripocalpida in the Plectoidea. Although such a monophyletic derivation of the Nassellaria from the Cyrtocalpida is possible, and though here, too, the Cortinida play an important part as connecting links, this hypothesis has less internal probability than that of the derivation from the Stephoidea (§ 184) or Plectoidea (§ 183).
186. Genealogical Tree of the Plectoidea.—The order Plectoidea includes those Nassellaria whose rudimentary skeleton does not contain the characteristic sagittal ring of the Stephoidea, but consists of several (at least three) radial spines, which proceed from a point in the centre of the porochora. The branches of these radial spines remain free in the Plagonida, whilst in the Plectanida they unite with each other to form a loose meshwork (not, however, a complete lattice-shell). The number and arrangement of the radial spines, which serve for generic distinctions, are the same in both families, so that each genus of the Plectanida has arisen from a corresponding genus of the Plagonida. The simplest Plagonida, which possess a basal tripod (Triplagia or Plagiacantha with three rays, Tetraplagia with four rays) are probably to be regarded as forming the common origin of the whole order. These agree with certain three- and four-rayed skeletal pieces of the Beloidea (Thalassosphærida and Sphærozoida); and also the four and six-rayed twinned pieces of the latter (spicula bigemina and trigemina) repeat in the same fashion the skeleton of the former (Plagonidium, Plagonium). This similarity, however, is a mere analogy and possesses no phylogenetic significance. On the other hand, certain Plagonida (Plagoniscus, Plagiocarpa), and the corresponding genera of Plectanida (Plectaniscus, Periplecta) seem to have important phylogenetic relations to certain Stephoidea (Cortina, Cortiniscus, &c.); the sagittal ring of the latter having perhaps arisen by the vertical apical spine of the former having been connected with their horizontal basal rod by two ventral apophyses growing out opposite to each other (compare pp. 902, 914, Plagiocarpa procortina, Pl. 91, fig. 5). In this case the Plectanida would belong to the simplest stem-forms of the Nassellaria.
187. Genealogical Tree of the Stephoidea.—The order Stephoidea includes all those Nassellaria whose skeleton does not form a complete lattice-shell, but consists of one or more rings, and often of a loose meshwork which arises by the union of branches of the rings. A vertical sagittal ring is constantly present, embracing the central capsule in the median sagittal plane, and forming at its basal pole various processes, the starting point for other skeletal forms. The most important of these is the tripodal Cortina
