Page:Scientific results HMS Challenger vol 18 part 2.djvu/818

1694

Definition.— with panelled shell, composed of polygonal plates. The shell is usually polyhedral, more rarely spherical, and the radial spines are usually (or perhaps constantly) branched and regularly arranged.

Definition.— with a spherical or regularly octahedral shell, composed of eight congruent, triangular plates, with six corners from which arise six radial spines, opposite in pairs in three diameters, perpendicular one to another.

The genus Circoporus, the simplest among the Circoporida, is distinguished by the regular octahedral form of the shell, with the three equal axes of the regular crystalline system perpendicular one to another. Six equal radial spines, arising from the six corners, lie opposite in pairs in those three dimensive axes. The eight equal triangular faces of the octahedron are sometimes plane, sometimes concave or convex, and sometimes the shell becomes spherical. In this case it becomes very similar to the Hexastylida among the.

1. Circoporus sexfurcus, Haeckel (Pl. 117, fig. 5).

Challengeria sp., John Murray, 1876, Proc. Roy. Soc. Lond., vol. xxiv. pl. xxiv. fig. 5.

Shell spherical, covered with irregular, polygonal plates. Six radial spines shorter than the diameter of the shell, covered with thin curved bristles, in the proximal half cylindrical, in the distal half forked, each with two equal curved fork-branches. Around the ciliated base of each spine a