CXX THE VOYAGE OF H.M.S. CHALLENGER. 



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 what- 

 ever, 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 arrange- 

 .ment 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 (Thalassosphserida and Sphserozoida) ; 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 



