REPORT ON THE RADIOLARIA. 907 



Radiolarien (Plagiacantha ahietina). Upon these two species the latter founded his 

 family Plagiacauthida, a term which was afterwards employed by Biitschli and others, 

 for the whole group of P 1 e c t o i d e a. Many new forms are contained in the collec- 

 tion of the Challenger, so that we may describe here nine genera and thirty-four 

 species. 



The family Plagonida may be divided into four different subfamilies, according to 

 the numbers of the radial spines which compose the skeleton : Triplagida with three, 

 Tetraplagida with four, Hexaplagida with six, and Polyplagida with numerous (seven to 

 nine or more) radial spines. These are united commonly in one common central point, 

 upon which rests the basal pole of the central capsule, with the porochora. More rarely 

 (in the genera Plagonidium, Plagiocarpa, and Plagonium) the spines arise in two 

 opposite groups (each with two or three spines) from the two poles of a common central 

 rod; in this case the basal pole of the central capsule with the porochora rests upon 

 the horizontal common rod, which corresponds probably to the basal part of the sagittal 

 ring of the Stephoidea and C y r t e 1 1 a r i a. 



The different forms which the skeleton of the Plagonida assumes in the different 

 genera of this family, and the important relations which these exhibit on the one hand to 

 the spicula of the Beloidea, and on the other hand to the shell of some Stephoidea 

 [Cortina, Cortiniscus, &c.) and Cyrtoidea (Pteroscenium, Clathrocorys, &c.), have 

 Ijeen already pointed out in the preceding description of the suborder Plectoidea. 

 There it is also demonstrated, that all these different forms may be derived from the 

 simplest tru'adial forms, Triplagia and Plagiacantha (compare above, pp. 900—904). 



Whilst the genera of the Plagonida are characterised by the number of the radial 

 spines and the peculiar mode of junction in a common central point or at the two poles of a 

 common central rod, the different species of this family may be defined by the peculiar form 

 of the spines and their branches. These morphological characters have also been already 

 described above. We repeat here only that the radial spines in the majority of species 

 are three-sided prismatic and verticillate, each verticil commonly with three branches. 

 The distal ends of these branches remain constantly free, and are never united, as is 

 always the case in the following family. 



The Central Capsule of the Plagonida exhibits the general characters of all 

 MoNOPYLEA (compare above, p. 890). It is commonly ovate or ellijjsoidal, with vertical 

 main axis ; on the lower pole of the latter is the porochora (or the " area porosa," from 

 which all pseudopodia radiate). This is in immediate connection with the central point 

 or central rod of the skeleton, in which' its radial spines are united. The topographical 

 relation of the supporting skeleton to the central capsule seems to exhibit in the 

 different genera of the Plagonida remarkable differences, as already demonstrated above 

 (p. 905). 



