Page:Scientific results HMS Challenger vol 18 part 1.djvu/387

Rh dimensive axes = 1 : 2 : 4. The radial proportion of the three pairs of spines = 1 : 3 : 8. Spines thin cylindrical, at the base angular.

Dimensions.—Diameter of the major shell axis 0.2, middle 0.1, minor 0.05; length of the major spines 0.3, middle 0.12, minor 0.04.

Habitat.—Indian Ocean, Madagascar, Rabbe, surface.

2. Hexastylidium spirale, n. sp.

Shell thick walled, spherical, thorny, with irregular, roundish pores, three to five times as broad as the bars; six spines very stout, prismatic (as broad as one large pore), with three ring-like, spirally convoluted edges. Two opposite major spines of extraordinary length, ten to twelve times as long as the shell diameter, the two middle opposite spines being about as long as the latter, the two minor scarcely one-third as long. (Similar to Hexastylus spiralis, Pl. 21, fig. 7, but distinguished by the very unequal length of the spines.)

Dimensions.—Diameter of the shell 0.12; length of the major spines 1 to 1.5 mm., middle 0.15, minor 0.04.

Habitat.—Central Pacific, Station 266, surface.

Definition.— with two concentric spherical lattice-shells.

Definition.— with two concentric lattice-spheres and six simple spines of equal size.

The genus Hexalonche is the most simple form, and probably the common ancestral form, of all Hexalonchida, or those Cubosphærida which possess two concentric latticed spheres, connected by six radial beams. Commonly one shell is intracapsular (medullary shell) and the other extracapsular (cortical shell); but sometimes also both shells are extracapsular, and these forms may perhaps be better separated as a peculiar genus Hexadilemma. In Hexalonche all six simple spines are of equal size, and opposite by pairs in three equal dimensive axes, corresponding to the three equal axes of a tesseral crystal. It can be derived from Hexastylus by duplication of the lattice-shell.

Definition.—Pores of the cortical shell regular or subregular, of nearly equal size and similar form; surface smooth, without radial by-spines (other than the six main spines).