Less important than those quadriradial Tetraplagida and Tetraplectida, are the sexradial #Plectoidea#, the Hexaplagida and Hexaplectida. These may be derived immediately from the triradial #Plectoidea# by prolongation of the three primary original spines (of _Plagiacantha_) over the common central point. Here also two {903}different kinds of central junction are found. In the simpler case all six radial spines arise from a common central point (_Hexaplagia_ and _Hexaplecta_). In the other case the six radial spines arise from the two poles of a short horizontal common central rod, opposed in two groups, each of three spines (_Plagonium_ and _Plectanium_, Pl. 91, figs. 6, 11). In this latter case the single corresponding spines of the two opposite groups are usually parallel, and exhibit therefore exactly the same characteristic "germinate-triradiate" form which is found in many #Beloidea# (_e.g._, in the common _Sphærozoum punctatum_ and the similar _Lampoxanthium punctatum_). The fourth and last group of this suborder contains the multiradiate #Plectoidea#, the Polyplagida and Polyplectida. Here the number of radial spines, diverging from the common centre, exceeds six, and is commonly seven to nine, at other times ten to twelve or more (_Polyplagia_ and _Polyplecta_, Pl. 91, fig. 12). When these two genera are better known from further accurate observations, they may probably be divided into several different genera (as already proposed in my Prodromus, 1881), since not only the number, but also the central junction and the arrangement of the numerous radial spines in the few observed species seems to be very different. In some seven-radiate species (_e.g._, _Polyplecta heptacantha_) four larger spines seem to be true "cortinar spines," the three smaller secondary productions of the former. In the nine-radiate species the nine spines seem to be sometimes basal branches of three primary spines, at other times six secondary intercalated between the three primary (like _Enneaphormis_, Pl. 57, fig. 9). In those multiradial #Plectoidea#, in which the number of spines amounts to ten or twelve or more, the laws of disposition are not yet recognised.