The four spines, which we regard therefore as "cortinar spines," exhibit a twofold kind of central junction. In the simpler case they are united in a common central point, on which rests the porochora of the central capsule (_Plagoniscus_ and _Plectaniscus_, Pl. 91, figs. 4, 9). These forms are nearer to _Cortina_, and may be derived immediately from _Tetraplagia_ and _Tetraplecta_ by differentiation of the four equal spines. In the other case the four cortinar spines are separated in pairs, diverging from the two poles of a short horizontal common "central rod" (_Plagiocarpa_ and _Periplecta_, Pl. 91, figs. 5, 10). These forms may be compared with the spicula of some #Beloidea# and derived from _Plagonidium_; but their basal central rod may be compared again with the basal part of the sagittal ring of _Cortina_, and this comparison becomes very important in those forms like _Plagiocarpa procortina_ (Pl. 91, fig. 5). Here the four spines approach very nearly to those of _Cortina_; the two ventral spines (or pectoral feet) on the anterior pole of of the middle rod are equal, but very different from the two dorsal spines, arising from the posterior pole; the lower odd spine of the latter corresponds to the "caudal foot," the upper spine to the "apical horn" of _Cortina_ and of the #Cyrtellaria#. The vertical plane, determined by these two dorsal spines, is the sagittal plane, and two opposite curved branches which lie in this plane (an upper arising from the basal part of the apical spine and a lower arising from the anterior pole of the middle rod) may be regarded as ventral parts of an incomplete sagittal ring. This interesting form and some other similar Tetraplagida may be regarded either as beginning #Stephoidea# (_Cortina_, with incomplete sagittal ring) or as retrograde #Stephoidea# (_Cortina_, with partly reduced sagittal ring). In every case they seem to indicate the near relationship between the #Stephoidea# and #Plectoidea#. Another argument for this close relationship may be found in the position of the central capsule in the interesting genus _Plagiocarpa_ (Pl. 91, fig. 5). Its basal part (with the porochora) rests upon the common central rod, its ventral face upon the ventral prolongation of the latter, its dorsal face upon the apical horn; its axis lies in the sagittal plane. The three basal spines (the odd caudal and the paired pectoral feet) diverge from its basal pole downwards in the same manner as in the Cortina, the Zygospyrida and the Monocyrtida.