Comparing these different productions of the skeleton in the numerous #Plectoidea#, we find expressed two remarkable and very different affinities. On the one hand many #Plectoidea# exhibit exactly the same peculiar forms, which are only found besides in the #Beloidea# (as many species of _Triplagia_, _Plagiacantha_, _Tetraplagia_, _Plagonidium_, _Hexaplagia_, and _Plagonium_). On the other hand many #Plectoidea# bear the same characteristic composition of the skeleton (or the "cortinar structure") which is found in the Cortinida among the #Stephoidea#, and in numerous #Spyroidea# and #Cyrtoidea#, which all agree in the possession of three divergent basal feet and a vertical apical horn. A most important argument for the close affinity of all these "cortinar NASSELLARIA" seems to be given by the fact that the sagittal ring, which in _Cortina_ is combined with the quadriradial structure, exhibits in the #Cyrtellaria# the most different stages of development; in one group it is complete, in the second incomplete, and in the third it has completely disappeared. The form of the radial spines composing the skeleton is usually three-sided prismatic, gradually tapering from the thicker central base towards the distal apex; sometimes they are slender pyramidal. More rarely the spines are cylindrical or slender conical. In the majority of species the spines are straight, in the minority more or {904}less curved. In very few species only are they quite simple, without branches. They are nearly always more or less branched, in many larger species very richly ramified. The modes of ramification are rather variable. In the majority of #Plectoidea# the spines are rather regularly verticillate, bearing an increasing number of verticils, each of which is composed of three divergent branches. These arise from the three edges of the spine, and all the branches of one edge are usually parallel, either perpendicular to the spine, or directed at an acute angle towards its apex. When the verticils are numerous (five to ten or more), their size commonly tapers gradually towards the apex. Pinnate spines occur more rarely than verticillate ones; in this case the two paired lateral edges only of the prismatic spine bear opposite or alternate branches, whilst the odd middle edge bears no ramules. In some species the spines are singly or doubly forked. In many species (mainly those with cylindrical spines) the ramification of the spines is more or less irregular.