Page:Scientific results HMS Challenger vol 18 part 1.djvu/878

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Definition.— with diaxial growth; cupolas crossed in pairs, opposite on the poles of two axes, perpendicular one to another. (Shell commonly five-chambered, with four domes, cross-wise surrounding the central chamber.)

Definition.— with simple cortical shell (without external veil), composed of four hemispherical cupolas in cross-form, opposite in pairs on the poles of two axes perpendicular one to another; central chamber simple (without medullary shell).

The genus Tholostaurus is the most simple form of the Staurotholida, or the Tholonida with four crossed hemispherical cupolas, lying on the poles of two axes perpendicular one to another. The central chamber communicates by four wide openings with the four domes, and contains no medullary shell. Tholostaurus may originate either from Tholartus by apposition of two opposite cupolas between the first pair, or from Staurotholus by loss of the medullary shell.

Definition.—Surface of the shell smooth or rough, without radial spines.

1. Tholostaurus quadrigatus, n. sp.

All four cupolas nearly of the same size and form, subregular. Surface smooth, without radial spines. Pores subregular, circular, four times as broad as the bars; eight to ten pores in the basal semicircle of one cupola.

Dimensions.—Diameter of the shell 0.12; pores 0.008, bars 0.002.

Habitat.—Pacific, central area, Station 272, surface.

2. Tholostaurus cruciformis, n. sp.

Two opposite cupolas larger than the two others. Surface rough, without radial spines. Pores regular, circular, hexagonally framed, twice as broad as the bars; twelve to fourteen in the basal semicircle of one cupola.

Dimensions.—Major axis of the shell 0.16, minor axis 0.12; pores 0.006, bars 0.003.

Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.