The triradial structure, common to the Triplagida and Triplectida, is replaced by the quadriradial structure in the Tetraplagida and Tetraplectida. Probably the latter have been derived from the former by development of a fourth spine, and then this latter would correspond to the "apical horn" of the other NASSELLARIA. But possibly also both structures have originated independently from one another. We may distinguish not less than four different kinds of the quadriradial structure. In the first case all four spines are equal, and diverge from a common central point at equal angles in different directions, corresponding to the four axes of a regular tetrahedron (_Tetraplagia_ and _Tetraplecta_, Pl. 91, figs. 3, 8). In the second case all four spines are also equal, but they are not united in a common central point, but opposite in pairs on the two poles of a common central rod (_Plagonidium_). Therefore the skeleton possesses here the same form as in the "geminate-biradiate" spicula of many #Beloidea# (_e.g._, _Thalassoxanthium bifurcum_ and _Sphærozoum furcatum_). The development of the short horizontal middle rod, connecting the two divergent pairs of spines, is here probably effected by the porochora of the central capsule resting upon it. Whilst in these two cases of quadriradial structure all four spines are equal, in two other cases they become differentiated in a very remarkable manner. One spine is vertically directed upwards, in shape and size different from the three others, which are directed downwards; the former corresponding probably to the "apical horn," the latter to the three "basal feet," which are found in the great majority of the #Spyroidea# {902}and #Cyrtoidea#. Therefore we encounter here for the first time that characteristic "cortinar structure" which is complete in _Cortina_ and _Cortiniscus_ (Pl. 92, figs. 11-13, 21), and which may be regarded as the strongest argument for a close relationship, or even for a common monophyletic origin of all NASSELLARIA.