Page:Scientific results HMS Challenger vol 18 part 1.djvu/841

Rh

Definition.— with simple, spherical or subspherical, central chamber, surrounded by one single (transverse) latticed girdle.

The genus Monozonium is the most simple and primitive of all Pylonida, and may be regarded as their common ancestral form; it may probably be derived phylogenetically either directly from Cenosphæra or from Lentellipsis, either by apposition of two imperfect lateral chambers, or by surrounding it with an equatorial latticed girdle. This transverse girdle is composed of three parts, the central chamber and two lateral wings, which represent two short hollow latticed tubes, the axes of which are parallel to the principal axis. On both principal sides (on the anterior and posterior faces) there are two large open gates as in Amphipyle. If we imagine the openings of the tube-shaped lateral wings closed by lattice-work, Monozonium becomes transformed into Tholartus, the most simple form of Tholonida. Probably in all Pylonida the ontogeny of the shell begins with the formation of a Monozonium.

Definition.—Shell smooth or rough, without radial spines or thorns.

1. Monozonium primordiale, n. sp.

Central chamber of the shell spherical, smooth, with three to four pores on the half equator. Both wings of the girdle of the same breadth, but of twice the length of the central chamber, with three to four longitudinal rows of pores. No radial spines.

Dimensions.—Diameter of the central chamber 0.02; breadth of the wings 0.02, length 0.04; pores and bars 0.003.

Habitat.—Pacific, central area, Station 271, surface.

2. Monozonium alatum, n. sp. (Pl. 9, fig. 1).

Central chamber of the shell lentelliptical, smooth, one and a half times as long as broad, with three to four pores on the half equator. Both wings of the girdle twice as broad, and somewhat longer than the central chamber, with five to six longitudinal rows of pores. No radial spines.

Dimensions.—Length of the central chamber 0.03, breadth 0.02; breadth of the wings 0.04, length 0.05; pores and bars 0.004.

Habitat.—North Pacific, Station 241, surface.