The different genera of Plectanida, which are enumerated in the following synopsis, correspond so exactly to the different genera of Plagonida described above, that probably each of the former has arisen from a corresponding genus of the latter. The only difference between the two is, that in all Plagonida the branches and ramules of the radial spines end freely, without growing together, whilst in all Plectanida the meeting ends of the neighbouring spines grow together and so produce a loose and irregular wickerwork. Therefore the latter constantly possess meshes, which are missing in the former. As soon as any form of Plagonida begins to produce meshes by concrescence of meeting branches of the spines, it passes over into a corresponding form of Plectanida. The loose wickerwork or wattlework so produced is sometimes very irregular, at other times more or less regular; but it never assumes the regular form of a complete lattice-shell, as in the #Cyrtellaria# (#Spyroidea#, #Botryodea#, and #Cyrtoidea#). The general differences which this wickerwork exhibits in the different genera of Plectanida have been already described above (compare pp. 900-904). _The Central Capsule_ of the Plectanida agrees perfectly with that of the ancestral Plagonida, as well in general form and structure as in the peculiar topographical relation to the radial spines (compare above, p. 905). A slight difference between the two families is effected by the higher development of the skeleton in the Plectanida. Since the branches of the radial spines in this family become united and form a loose wickerwork, the central capsule becomes more or less enclosed by the latter, and more perfectly protected, than in the Plagonida, where the branches remain free. In this respect the Plectanida approach more nearly to the #Cyrtoidea# with which they are connected by such transitional forms as _Plectaniscus_ and _Periplecta_ on the one hand, _Cladoscenium_ and _Pteroscenium_ on the other. {921}_Synopsis of the Genera of Plectanida._ ------------------------------------------------------------------------- I. Subfamily Triplectida. Three radial spines. ------------------------------------------------------------------------- Three spines lying in one horizontal plane, 393. _Triplecta_. Three spines corresponding to the edges of a flat pyramid, 394. _Plectophora_. ------------------------------------------------------------------------- II. Subfamily Tetraplectida Four radial spines. ------------------------------------------------------------------------- { All four spines equal, 395. _Tetraplecta_. Four spines arising from { one common central { One apical spine opposed point. { to three basal spines, 396. _Plectaniscus_.