183. _Ascent of the Nassellaria from the Plectoidea._--The monophyletic hypothesis (No. 2, p. 893) which regards the basal tripod as the common origin of the skeleton of all NASSELLARIA, starts from the simplest forms of the #Plectoidea# (_Triplagia_, _Plagoniscus_, _Triplecta_, _Plectaniscus_, &c., Pl. 91). All #Plectoidea# may be immediately derived as diverging twigs of these, as well as all triradial and multiradial forms of #Cyrtoidea# and #Spyroidea#; for in all these cases the distinctive triradial (or the derived multiradial) form of skeleton appears directly derivable from the simple basal tripod of the former. The same is perhaps also true of many #Botryodea#. {cxix}Furthermore, certain important forms of #Stephoidea# (_Cortina_, _Cortiniscus_, _Stephanium_, _Stephaniscus_, &c.), which have a characteristic combination of the sagittal ring and basal tripod, may be immediately derived from such forms of #Plectoidea# as _Plagoniscus cortinaris_, _Plagiocarpa procortina_, _Plectaniscus cortiniscus_, &c. On the contrary, those #Stephoidea# and #Cyrtoidea# in which the basal tripod is wanting can only be derived from the #Plectoidea# by the assumption that this structure has disappeared in consequence of phylogenetic degeneration. The monophyletic derivation of the NASSELLARIA from the #Plectoidea# has more internal probability than that from the #Stephoidea#, since it is easier to suppose that the Cortinida (_Cortina_, _Stephanium_, &c.) have been derived from the #Plectoidea# (_Plagoniscus_, _Plagiocarpa_) than the converse. This view is the basis of the hypothetical tree shown in § 180. 184. _Ascent of the Nassellaria from the Stephoidea._--The monophyletic hypothesis (No. 1, p. 893) which regards the primary sagittal ring as the common starting point of the skeleton in all NASSELLARIA, starts from the simplest forms of #Stephoidea# (_Archicircus_, _Lithocircus_, &c., Pl. 81). All #Stephoidea# and #Spyroidea# may be immediately derived from these, as also the majority of the #Cyrtoidea# and probably of the #Botryodea#. Those numerous forms of the last two groups, however, which possess no trace of a sagittal ring, can only be derived from the former by the supposition that the latter has completely disappeared in in consequence of gradual phylogenetic degeneration. The same holds true also of the #Plectoidea#, although certain forms (_e.g._, _Plagiocarpa procortina_, Pl. 91, fig. 5; _Plectaniscus cortiniscus_, Pl. 91, fig. 9) appear to indicate the commencing formation of the sagittal ring by the concrescence of two branches, which approach each other from the upper part of the apical rod and the ventral part of the basal rod. In any case, it is a fact of great phylogenetic significance, that the primary sagittal ring in the cephalis of the #Cyrtoidea# shows all conceivable stages of degeneration (compare Bütschli, L. N. 40, 41, as well as the general account of and critical comparison of the NASSELLARIA, pp. 889-895, &c.).