The genus Cœlothauma differs from the preceding Cœlotholus, its ancestral form, in the possession of twelve long, prominent styles, six of which are opposite in three pairs on each valve. In the single species observed each of the four main tubes (opposite in pairs on the frontal corners of the two galeæ) is simply forked, as in Cœlotholus; but whilst in this latter all eight styles remain simple, in Cœlothauma the anterior (or pectoral) branch only is simple, the posterior (or tergal) branch is again forked.
1. Cœlothauma duodenum., n. sp. (Pl. [122], figs. 3-5).
Twelve styles straight, of different lengths. Two short bilateral main tubes arise opposite in pairs from the two frontal corners of each galea, and are divided into an anterior and posterior branch. The anterior or pectoral branch is simple, and twice as long as the diameter of the fork-thicket. The posterior or tergal branch is again forked near the base, and its two divergent branches are three times as long as the diameter of the fork-thicket. All twelve styles are densely studded with anchor-pencils of nearly equal size.
Dimensions.—Diameter of the whole body 21, of the fork thicket 3.5.
Habitat.—South-Western Pacific (east of New Zealand), Station 169, depth 700 fathoms.
Genus 733. Cœlothamnus,[[349]] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.
Definition.—Cœlographida with two paired lateral frenula on each galea, without external lattice-mantle, armed with sixteen styles (four pairs of styles on each valve).
The genus Cœlothamnus differs from its ancestral form, Cœlotholus, in the duplication of the number of long verticillate styles, which are prominent over the surface of the dense fork-thicket. Whilst in Cœlotholus each frontal main tube (arising from the frontal corner of the galea at right and left) is divided into an anterior or pectoral, and a posterior or tergal style, in Cœlothamnus each of these two divergent main styles is again forked, so that the total number of projecting and radially diverging styles amounts to sixteen. One Mediterranean species of this genus, Cœlothamnus davidoffii, has been already described by Bütschli in 1882 (Zeitschr. f. wiss. Zool., vol. xxxvi. p. 486, Taf. xxxi.). Though its description on the whole is accurate, some important errors, which may be here corrected, are to be met with. The two central valves of the lattice-shell (dorsal and ventral) are symmetrically equal in size and in form, as in all other Cœlographida; the different forms and the inverse arrangement of the two valves, described by Bütschli (pp. 488, 491), were effected by an artificial dislocation and inversion. The peculiar opening α, which, according to his opinion, was supposed to bring about a direct communication between the cavities of the galea and its valve, is in reality the optical section of the rhinocanna, the two convergent frenula of which (γ) he figured, but did not recognise. Cœlothamnus attains the greatest size among all Radiolaria; the diameter of the body in Cœlothamnus maximus amounts to 33 mm.
1. Cœlothamnus bivalvis, n. sp. (Pl. [122], figs. 6-9).
Sixteen styles all of equal length, about three times as long (in their free part) as the diameter of the dense fork-thicket. The size of the anchor-pencils tapers from the proximal to the distal end. Each of the four primary frontal tubes (which arise in opposite pairs from the frontal corners of the two galeæ) is twice forked, and so produces four styles.