REPORT ON THE RADIOLARIA. 899 



The suborder Plectoidea, hitherto known by few species only of "Plagiacanthida," 

 comprises a large numlier of interesting Nassellaria, 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 dilier from the following suborder, the S t e p h o i d e a, 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 Claparede, and described 

 and figured in his Etudes, &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 Miiller as Acanthodesmia 

 dumetum (1858, loc. cit., Ta£ i. fig. 3). A third species, also Mediterranean, was 

 figured by me in 1865 under the name Acanthodesmia polyhroclia. Finally, Richard 

 Hertwig, 1879, in his Organismus der Radiolarien, gave a very accurate description 

 of another Mediterranean form, Plagiacantha ahietina (loc. cit., Taf. vii. fig. 6). He 

 first reco2;nised 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 Biitschli). But at present, regarding the 

 important relations of these Plectida to the other Nassellaria, it seems more con- 

 venient to give to them the rank of an independent suborder of Radiolaria, under the 

 name Plectoidea. 



The peculiar structure of the central capsule of the Plectoidea, first recognised 

 by Richard Hertwig, allows no doubt of their being true Monopylea or Nassellaria ; 

 and also their siliceous, originally trii'adiate skeleton indicates the nearest affinity to 



