{899}The suborder #Plectoidea#, hitherto known by few species only of "Plagiacanthida," comprises a large number of interesting NASSELARIA, which belong partly to the simplest and most primitive forms of this legion. It may be divided into two different families, Plagonida and Plectanida. In the first family, Plagonida, the monopylean central capsule is supported by a simple or rudimentary skeleton, composed only of a variable number of radial spines united in a common centre. In the second family, Plectanida, the branches of these radial spines become united and form a loose irregular framework with wide meshes, partly enclosing the central capsule, but never a perfect lattice-shell. The #Plectoidea# differ from the following suborder, the #Stephoidea#, in the absence of the ring, characteristic of the latter. Some slight traces, however, indicate a near affinity between the ringless #Plectoidea# and the ring-bearing #Stephoidea#. Both these suborders of #Plectellaria# differ from the closely allied #Cyrtellaria# (#Spyroidea#, #Botryodea#, and #Cyrtoidea#) in the absence of a complete lattice-shell. The morphological relation and phylogenetic affinity between the former and the latter have already been discussed in the preceding description of the legion NASSELLARIA (compare pp. 891-894). The first known species of #Plectoidea# was observed in the North Atlantic (on the Norwegian shore) in 1855 by my late friend Edouard Claparède, and described and figured in his Études, &c. (1858), under the name _Plagiacantha arachnoides_. He considered it as a new genus of Acanthometrina. Another species, from the Mediterranean, was described in the same year by Johannes Müller as _Acanthodesmia dumetum_ (1858, _loc. cit._, Taf. i. fig. 3). A third species, also Mediterranean, was figured by me in 1865 under the name _Acanthodesmia polybrocha_. Finally, Richard Hertwig, 1879, in his Organismus der Radiolarien, gave a very accurate description of another Mediterranean form, _Plagiacantha abietina_ (_loc. cit._, Taf. vii. fig. 6). He first recognised the true character of MONOPYLEA in their monaxonian central capsule, and observed at the same time the first NASSELLARIUM without skeleton, called by him _Cyrtidium inerme_ (_loc. cit._, Taf. vii. fig. 1). To these four known species, representing three different genera, the rich collection of the Challenger has added so many new forms that we may distinguish here not less than seventeen genera and sixty-one species. In my Prodromus (1881, p. 423) I arranged these in two subfamilies, the Plagonida and Plectanida, constituting together the family Plectida (identical with the "Plagiacanthida" of Hertwig and Bütschli). But at present, regarding the important relations of these Plectida to the other NASSELLARIA, it seems more convenient to give to them the rank of an independent suborder of Radiolaria, under the name #Plectoidea#.