REPORT ON THE RADIOLARIA. 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 " geminate- 

 triradiate " form which is found in many Beloidea (e.g., in the common Sphcerozoum 

 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. 



Comparing these different productions of the skeleton in the numerous Plectoidea, 

 we find expressed two remarkable and very different affinities. On the one hand many 

 Plectoidea exhibit exactly the same peculiar forms, which are only found besides 

 in the Beloidea (as many species of Triplagia, Plagiacantha, Tetraplagia, 

 Plagonidium, Hexaplagia, and Plagonium). On the other hand many Plectoidea 

 bear the same characteristic composition of the skeleton (or the " cortinar structure ") 

 which is found in the Cortinida among the Stephoidea, and in numerous 

 Spyroidea and Cyrtoidea, which all agree in the possession of three divergent 

 basal feet and a vertical apical horn. A most important argument for the close affinity 

 of all these " cortinar NASSELLAKIA " seems to be given by the fact that the sagittal 

 ring, which in Cortina is combined with the quadriradial structure, exhibits in the 

 Cyrtellaria the most different stages of development; in one group it is complete, 

 in the second incomplete, and in the third it has completely disappeared. 



The form of the radial spines composing the skeleton is usually three-sided 

 prismatic, gradually tapering from the thicker central base towards the distal apex ; 

 sometimes they are slender pyramidal. More rarely the spines are cylindrical or slender 

 conical. In the majority of species the spines are straight, in the minority more or 



