Page:Scientific results HMS Challenger vol 18 part 2.djvu/805

Rh mouth of the shell, therefore placed in its main axis; it is usually quite hidden in the dark phæodium, which occupies the oral half of the shell-cavity, whilst the capsule lies in the aboral half. The large nucleus of the central capsule and its double membrane present the same characters as in the other.

Definition.— without radial main-spines, with a smooth mouth.

The genus Castanarium is the simplest form of all the Castanellida, and may be regarded as the common ancestral form of this family. The simple spherical shell has a smooth, toothless mouth and no larger radial spines; it is densely studded with the simple, radial bristles, which are common to all Castanellida. The shell therefore is very similar to those species of the Astrosphæride Acanthosphæra, which represent the subgenus Rhapidococcus (compare above, p. 210, Pl. 26, fig. 3); it differs from the latter in the possession of the shell-mouth, which is absent in all.

1. Castanarium darwini, n. sp.

Pores regular, circular, hexagonally framed, three times as broad as the bars. Radial bristles half as long as the radius.

Dimensions.—Diameter of the shell 0.3 to 0.4, of the pores 0.03.

Habitat.—South Atlantic (east of Patagonia), Station 318, depth 2040 fathoms.