EEPORT ON THE EADIOLARIA. CXIX 



more, certain importaut forms of Stephoiclea {Cortina, Cortiniscus, Stephanium, 

 Stephcmiscus, &c.), wliieli have a eliaracteristic combination of tlie sagittal ring and basal 

 tripod, may be immediately derived from si;ch forms of Fleet oidea as Plagoniscus 

 cortinaris, Plagiocarpa pjrocortina, Plectaniscus cortiniscus, &c. On the contrary, those 

 S t e p h o i d e a and C y r t o i d e a in which the basal tripod is wanting can only be 

 derived from the P 1 e c t o i d e a 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 (Cor^«V(a, 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. 89-3) 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 S t e p h o i d e a and S p y r o i d e a may be 

 immediately derived from these, as also the majority of the C y r t o i d e a and probably of 

 the Botryodea. Those numerous forms of the last two groups, however, w^hich 

 possess no trace of a sagittal ring, can only be derived from the former by the supposi- 

 tion that the latter has completely disappeared in consequence of gradual phylogenetic 

 degeneration. The same holds true also of the Plectoidea, although certain forms 

 {e.g., Plagiocarpa procortina, PI. 91, fig. 5 ; Plectaniscus cortiniscus, PI. 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 C y r t o i d e a shows all conceivable 

 stages of degeneration (compare Biitschli, L. N. 40, 41, as well as the general account of 

 and critical comparison of the Nassellaria, pp. 889-895, &c.). 



185. Ascent of the Nassellaria from the Cyrtoiclea. — The monophyletic hypothesis 

 (No. 3, p. 894) which regards the latticed cephalis as the common point of origin of all 

 the skeletons of the Nassellaria, starts from the simplest forms of the C y r t o i d e a, that 

 is, from the Cyrtocalpida or eradial Mouocyrtida {Archicorida, Archicapsida , Pis. 51, 

 52, 98). All C y r 1 i d e a and Botryodea may be regarded as divergent forms of 

 these monothalamous C 5^ r t o i d e a ; the polythalamous simply liy the addition of fresh 

 joints at the basal pole, the triradiate and multiradiate by the development of three or 

 more apophyses. The origin of the sagittal ring (which presents every stage of develop- 

 ment and degeneration in the C y r 1 i d e a) may be regarded as a mechanical thickening 

 of the latticed plate in the sagittal circumference of the cephalis. By stronger develop- 



