Page:Scientific results HMS Challenger vol 18 part 1.djvu/694

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Definition.— with a simple central chamber, surrounded by a single concentric ring, which is divided by radial beams into two to six or more radial chambers, without radial spines on the margin.

The genus Archidiscus begins the long and polymorphous series of the or of those  which do not possess the peculiar "phacoid shell" characteristic of the three preceding families, united as "." As already mentioned above, both these groups are probably of independent origin, derived from the in different ways (compare pp. 402, 405, &c.). Among all Archidiscus is the most simple, and probably the common ancestral form, from which the other genera may be derived.

Definition.—Ring with two chambers, separated by two radial beams.

1. Archidiscus dioniscus, n. sp.

Ring circular, connected with the central chamber by two radial beams, opposite in one axis, therefore two equal semicircular ring chambers. (This primitive form has an interesting reference to Saturnalis, Pl. 13, fig. 16, and differs from it only in the lattice-work covering both faces of the lenticular disk, the margin of which forms the ring.)

Dimensions.—Diameter of the ring 0.05, of the central chamber 0.016.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

2. Archidiscus dithalamus, n. sp.

Ring roundish, connected with the central chamber by two radial beams, not opposite in one axis; both semicircular ring chambers more or less unequal, one of them larger than the other, and sometimes much more prominent. (If this prominence increase, we can regard it as the beginning of spiral convolutions, Discospira.)

Dimensions.—Diameter of the ring 0.06, of the central chamber 0.014.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

Definition.—Ring with three chambers, separated by three radial beams.