The genus Oroplegma differs from the other Orosphærida in the development of an external lattice-work enveloping the internal primary shell, and produced by the union of the branches of the radial spines. This outer shell is either a simple lattice-plate (comparable to the cortical shell of Diplosphæra), or a spongy framework (as in Rhizoplegma).
Subgenus 1. Oroplegmium, Haeckel.
Definition.—External shell a simple fenestrated lamella, forming an outer concentric lattice-sphere around the inner primary shell.
1. Oroplegma diplosphæra, n. sp. (Pl. [107], fig. 1).
Radial spines slender, cylindrical, smooth, more or less curved, about twice as broad as the smooth bars of the network. External shell a simple irregular lattice-sphere, with loose polygonal meshes, which are on an average three to four times as broad as the irregular meshes of the internal shell. The free prominent parts of the radial spines are irregularly branched, very long, with partly confluent branches.
Dimensions.—Diameter of the inner sphere 1.5 to 2.0, of the outer 2.5 to 3.0; length of the free spines 1.2, breadth 0.04.
Habitat.—Central Pacific, Station 265, depth 2900 fathoms.
2. Oroplegma spinulosum, n. sp.
Radial spines slender, cylindrical, thorny, more or less curved, very similar to those of the preceding species; the shell also much resembles that of Oroplegma perplexum. It differs from the latter in the more regular fenestration of both shells, and mainly in the shape of the bars and the spine-branches, which are all spinulate and densely studded with small conical thorns.
Dimensions.—Diameter of the inner sphere 1.5 to 2.0, of the outer 2.5 to 3.0; length of the spines 1.2, breadth 0.03.