Dimensions.—Major axis of the shell 0.15, minor axis 0.12; pores and bars 0.007.
Habitat.—Indian Ocean, Zanzibar, Pullen, depth 2200 fathoms.
Genus 294. Tholoma,[[333]] n. gen.
Definition.—Tholonida with double cortical shell (with external veil), composed of four hemispherical cupolas in cross-form, opposite in pairs on the poles of two axes perpendicular one to another; central chamber simple (without medullary shell).
The genus Tholoma (Pl. [10], figs. 10, 13) differs from the preceding Tholostaurus (its probable ancestral form) only in the duplication of the shell. The outer (secondary) shell has the same cross-form as the inner (primary) shell. In the two observed species the growth seems to be different, in the first species all four cupolas of each cross being of the same size, form, and age; in the second two opposite cupolas, larger and apparently older than the other two. Tholoma is possibly the offspring of Staurotholonium, from which it may have been produced by loss of the medullary shell.
Subgenus 1. Tholomantha, Haeckel.
Definition.—Surface of the shell smooth, without radial spines.
1. Tholoma quadrigeminum, n. sp. (Pl. [10], fig. 10).
All four chambers in each cortical shell nearly equal, subregular. Surface smooth, without radial spines. Structure of the network in both shells similar. Pores regular, circular, twice as broad as the bars; six to eight in the basal semicircle of one inner, ten to twelve in the semicircle of one outer cupola.
Dimensions.—Diameter of the outer shell 0.14, of the inner 0.09; pores 0.006, bars 0.003.