The important signification of the triradial structure, recurring in the most different groups of NASSELLARIA, has been already pointed out sufficiently by myself and by R. Hertwig. But the triradial #Plectoidea# offer also another interesting relation of this characteristic structure, some simple forms of this order appearing nearly identical with the isolated triradial spicula of certain #Beloidea# (Thalassosphærida and Sphærozoida). Even some more complex quadriradial and sexradial forms of the latter reappear in exactly the same shape also in the former. This identity may be perhaps an important indication of true affinity (compare below). The simplest and probably the most original kind of triradial structure is exhibited by the genera _Triplagia_ and _Triplecta_ (Pl. 91, figs. 2, 7). Here three equal radial spines lie in one horizontal plane and are united in a common central point at equal angles, so that three lines connecting their distal ends form a regular equilateral triangle. Simple triradial spicula of the same regular form are also found in many {901}#Beloidea# (_Lampoxanthium_, _Sphærozoum_, &c., Pls. 2 and 4). The central capsule of these simplest #Plectoidea# (with vertical main axis) rests perpendicularly on the horizontal triangle, formed by the triradial skeleton; the porochora of the former (or the "area porosa") rests upon the central point of the latter. Another kind of triradial structure characterises the genera _Plagiacantha_ and _Plectophora_. The three radial spines united in the central point lie here not in one plane, but diverge in different planes, so that they correspond to the three lateral edges of a three-sided pyramid. Commonly the three spines are of equal size, and also the angles between them equal, so that the pyramid is regular, sometimes very flat, at other times more elevated. Spicula of exactly the same form are also found in some #Beloidea#. Probably the three divergent spines are homologous to the three basal feet of numerous #Spyroidea# and #Cyrtoidea#. The central capsule, according to Hertwig, is placed in the apical part of the pyramid, the axes of both being identical, and the porochora resting in the apex itself. This fact seems to contradict the above-mentioned affinity; but since in _Triplecta_ and _Triplagia_ the three spines lie horizontally, they may have changed this original position in different direction, in _Plagiacantha_ and _Plectophora_ becoming divergent upwards, whereas in _Plagoniscus_ and _Plectaniscus_ (as in the #Spyroidea# and #Cyrtoidea#) directed downwards.