REPORT ON THE RADIOLARIA. Ixxxiii 



distinct traces of the sagittal ring. Hence the skeletons of all Nassellaria may be 

 derived monophyletically (Hypothesis A, p. 893) from a simple sagittal ring (Archicircus 

 and Lithocircus, PL 81). This theory, however, encounters the great difficulty that 

 in many S t e p h o i d e a (Cortina, Cortiniscus, &c.) it is combined in a remarkable manner 

 with the basal tripod of the Plectoidea, whilst in these latter it is entirely wantino- 

 (compare p. 894). 



125. The Plectoid Skeleton. — Those forms are distinguished as Plectoid in which 

 three, four, or more radial siliceous spines proceed from a common point, which lies 

 excentrically outside the central capsule and at the basal pole of its vertical allopolar 

 main axis. This peculiar tyjje of skeletal formation only occurs in the legion Nassel- 

 laria, and is specially characteristic of the order Plectoidea (p. 898, PI. 91). But 

 since the essential elements of this remarkable skeleton also occur in many other 

 Nassellaria, sometimes combined with the Circoid, sometimes with the Cyrtoid 

 skeleton, it perhaps has a fundamental significance in this legion ; at all events it is 

 possible to derive monophyletically all the other forms of this legion from it 

 (Hypothesis B, p. 893). The simplest form of the Plectoid skeleton is a tripod, the 

 three feet of which either lie in a horizontal plane {Triplagia, PI. 91, fig. 2), or 

 correspond to the three edges of a low pyramid {Plagiacantha). A fourth ray is sometimes 

 added, which stands vertically upon the summit of the pyramid [Plagoniscus, Plagiocarpa, 

 PI. 91, figs. 4, 5). In other Plectoidea three secondary rays are intercalated 

 between the three primary (Hexaplagida, &c.); seldom the number is greatly increased 

 (Polyplagida, &c.). The rays are rarely simple, but usually branched; in the Plagonida 

 (PI. 91, figs. 2-6) the branches remain free; in the Plectanida (PI. 91, figs. 7-13) they 

 are united to form a loose wicker-work. From such a web a perfect Cyrtoid shell may arise. 

 Several forms of Plagonida may also be readily confounded with the isolated triradiate 

 or quadriradiate spicula of many Beloid skeletons {Splicer ozouvi, Lampoxanthium, &c.). 



126. The S]3ongoid Skeleton. — From the simple lattice-skeleton which the majority 

 of Radiolaria possess, some of them develop a spongy shell ; the trabeeulse of the lattice- 

 work, situated in one plane in the former, are developed in the latter in difi'erent planes 

 and cross irregularly in all directions ; thus arises a kind of wicker-work of more or less 

 spongy structure, usually with very thin trabeculse and irregular meshes. Such Spongoid 

 shells are most common among the Spumellaria, especially in the Sphteroidea 

 (Spongosphserida, PI. 18) and Discoidea (Spongodiscida, Pis. 41-47), more rarely 

 in the Prunoidea and L a r c o i d e a. Lattice-work of similar spongy structure occurs 

 very seldom among the Nassellaria, e.g., in some Plectoidea (PL 91) and 

 Cyrtoidea [Spongocyrtis, Spongopyramis, Spongovielissa, &c. , PL ^Q, fig. 1 ; 

 PL 64, figs. 5-10, &c.). Among the Ph^odaria spongy skeletons are very rare; they 



