Page:Scientific results HMS Challenger vol 18 part 1.djvu/874

666 bars; six to seven in the semicircle of one inner cupola, ten to twelve in the semicircle of one outer cupola. Both shells are connected by eight radial beams, crossed in two diagonal planes.

Dimensions.—Major axis of the outer shell 0.1, minor 0.065; major axis of the inner shell 0.05, minor 0.04; pores 0.006, bars 0.002.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.

Definition.— with simple cortical shell (without external veil), with two hemispherical cupolas, opposite on the poles of one axis; central chamber (between them) Larnacilla-shaped, with medullary shell.

The genus Amphitholus is the most simple form of all Coccotholida (or all Tholonida provided with medullary shell). Its simple cortical shell exhibits, like Tholartus, two lateral hemispheric cupolas, attached on both sides of a lentelliptical central chamber (on the poles of its transverse axis). The central chamber has the form of a triaxial ellipsoid, and contains, like Larnacilla, a small medullary shell, connected with it by the wings of a latticed transverse girdle. Amphitholus may be derived either from Tholartus by secondary production of a cortical shell, or (more probably) from Larnacilla by apposition of two lateral dome-shaped protuberances. In the latter case it may be regarded as an Amphipyle, the lateral open wings of which are closed by lattice-work.

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

1. Amphitholus artiscus, n. sp. (Pl. 10, fig. 3).

Central chamber of the same height as both cupolas. Surface of the cortical shell rough. Pores regular, circular, with prominent hexagonal frames, twice as broad as the bars; eight to ten in the basal semicircle of one cupola.

Dimensions.—Major axis of the shell 0.16, minor axis 0.1; pores 0.008, bars 0.004.

Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.

2. Amphitholus artidium, n. sp.

Central chamber one-fourth higher than both cupolas. Surface of the cortical shell smooth. Pores irregular, roundish, once to three times as broad as the bars. (Very similar to Tholartus tricolus, Pl. 10, fig. 1, but different by the medullary shell.)

Dimensions.—Major axis of the shell 0.15, minor axis 0.12; pores 0.004 to 0.01, bars 0.003.

Habitat.—North Pacific, Station 241, depth 2300 fathoms.