185. _Ascent of the Nassellaria from the Cyrtoidea._--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 #Cyrtoidea#, that is, from the Cyrtocalpida or eradial Monocyrtida (_Archicorida_, _Archicapsida_, Pls. 51, 52, 98). All #Cyrtoidea# and #Botryodea# may be regarded as divergent forms of these monothalamous #Cyrtoidea#; the polythalamous simply by 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 development and degeneration in the #Cyrtoidea#) may be regarded as a mechanical thickening of the latticed plate in the sagittal circumference of the cephalis. By stronger {cxx}development 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).