Page:Scientific results HMS Challenger vol 18 part 1.djvu/697

Rh 11. Archidiscus hexathalamus, n. sp.

Ring irregular, roundish, or hexagonal, connected with the central chamber by six radial beams of unequal increasing length; therefore all six ring chambers of gradually increasing size (beginning a spiral convolution, original form of some Discospira).

Dimensions.—Diameter of the ring 0.05 to 0.07, of the central chamber 0.014.

Habitat.—South Pacific, Station 300, depth 1375 fathoms.

Definition.—Ring with seven or more chambers, separated by seven or more radial beams.

12. Archidiscus octoniscus, n. sp.

Ring circular or regular octagonal, connected with the central chamber by eight equidistant radial beams; therefore eight ring chambers of equal size. (Compare the central part of the disk of Porodiscus quadrigatus, Pl. 41, fig. 3.)

Dimensions.—Diameter of the ring 0.04, of the central chamber 0.014.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

13. Archidiscus polythalamus, n. sp.

Ring circular, connected with the central chamber by nine to ten or more radial beams at nearly equal distances. Ring chambers nine to ten, more or less equal, sometimes also eleven to twelve, more different. (This species is very variable, and may perhaps be divided into a number of different "transformistic species.")

Dimensions.—Diameter of the ring 0.04 to 0.06, of the central chamber 0.015.

Habitat.—Central Pacific, Stations 265 to 274, depths 2350 to 2925 fathoms.

Definition.— with a simple central chamber, surrounded by one single concentric ring, which is divided by radial beams into two to six or more radial chambers; margin of the disk armed with radial spines.

The genus Axodiscus differs from the preceding Archidiscus only in the shape of the margin of the small lenticular disk, which is armed with a variable number of radial spines, indicating certain axes or radii of the shell. If these marginal spines at certain equal distances from the margin branch and their distal ends become united by these branches forming a concentric second ring, the genus passes into Porodiscus. The different number and disposition of the marginal spines are probably very important,