EEPORT ON THE RADIOLARIA. 901 



Beloidea [Lampoxanthium, Splicerozoum, &c., Pis. 2 and 4). The central capsule 

 of these simplest Plectoidea (with vertical main axis) rests perpendicularly on the 

 horizontal triangle, formed by the triradial skeleton ; the porochora of the former (or 

 the " area porosa ") rests upon the central point of the latter. 



Another kind of triradial structure characterises the genera Plagiacantlia and 

 Plectop)hora. The three radial spines united in the central point lie here not in one 

 plane, but diverge in different planes, so that they correspond to the three lateral 

 edges of a three-sided pyramid. Commonly the three spines are of equal size, and also 

 the angles between them equal, so that the pyramid is regular, sometimes very 

 flat, at other times more elevated. Spicula of exactly the same form are also found in 

 some Beloidea. Probably the three divergent spines are homologous to the three 

 basal feet of numerous Spyroidea and C y r t o i d e a. The central capsule, accord- 

 ing to Hertwig, is placed in the apical part of the pyramid, the axes of both being 

 identical, and the porochora resting in the apex itself. This fact seems to 'contradict 

 the above-mentioned affinity ; but since in Trvplecta and Triplagia the three spines lie 

 horizontally, they may have changed this original position in different direction, in 

 Plagiacantha and Plectophora becoming divergent upwards, whereas in Plagoniscus 

 and Plectaniscus (as in the Spyroidea and Cyrtoidea) directed dowTiwards. 



The triradial structure, common to the Triplagida and Triplectida, is replaced by the 

 quadriradial structure in the Tetraplagida and Tetraplectida. Probably the latter have 

 been derived from the former by development of a fourth spine, and then this latter would 

 correspond to the "apical horn" of the other Nassellaria. But possibly also both struc- 

 tures have originated independently from one another. We may distinguish not less 

 than four different kinds of the quadriradial structure. In the first case all four spines 

 are equal, and diverge from a common central point at equal angles in different direc- 

 tions, corresponding to the four axes of a regular tetrahedron (Tetraplagia and 

 Tetraplecta, PI. 91, figs. 3, 8). 



In the second case all four spines are also equal, 'bnt they are not united in a 

 common central point, but opposite in pairs on the two poles of a com m on central rod 

 (Plagonidium). Therefore the skeleton possesses here the same form as in the 

 " geminate-biradiate " spicula of many Beloidea (e.g., Tlialassoxanthium bifurcum 

 and Sphmrozoum furcatum). The development of the short horizontal middle rod, con- 

 necting the two divergent pairs of spines, is here probably effected by the porochora of 

 the central capsule resting upon it. 



Whilst in these two cases of quadriradial structure all four spines are equal, in two 

 other cases they become differentiated in a very remarkable manner. One spine is 

 vertically directed upwards, in shape and size different from the three others, which are 

 directed dowTiwards ; the former corresponding probably to the " apical horn," the latter 

 to the three "basal feet," which are found in the great majority of the Spyroidea 



