Genus 309. Spironium,[1] n. gen.
Definition.—Lithelida with double, trizonal or Larnacilla-shaped medullary shell; cortical shell subspherical or lentelliptical, constructed of two crossed spirals, which arise from both lateral wings of one girdle (commonly the lateral girdle) and revolve in an opposite diagonal direction around the principal axis.
The genus Spironium differs in a very remarkable manner from all foregoing Lithelida, and is distinguished by a quite peculiar mode of growth. It is most nearly allied to Larcopyle, and may, like this, be derived from Amphipyle (or rather from Larnacilla, beginning to transform into Amphipyle). But whilst in Larcopyle one of the two wings of the lateral girdle overgrows the other in the direction of the transverse axis (turning around the principal axis), here in Spironium both lateral wings begin at the same time to grow out from the lateral sides of the Larnacilla-shaped medullary shell; the most remarkable thing is, however, that the direction of growth in the wings is diverse from the beginning: the left wing grows downwards and turns around the lower (aboral) pole of the principal axis, the right wing grows upwards and turns around the upper (oral) pole of the same axis. Thus both wings of the lateral girdle are crossed in diagonal axes, and with increasing growth one overgrows the other in the direction of these diagonals, so as to resemble the figure 8 in shape. The open gates remaining between the turnings of the girdle become afterwards closed on the surface by irregular lattice-work, and so the whole cortical shell assumes finally a spherical, ellipsoidal, or lentelliptical form. Its surface sometimes becomes covered with simple or branched radial spines. In the interior the eight characteristic egg-shaped gates of Octopyle are commonly (or constantly?) visible, two strong radial beams in the principal axis arising from the poles of the lentelliptical medullary shell.
Subgenus 1. Spironetta, Haeckel.
Definition.—Surface of the shell smooth or rough, but not with radial spines.
1. Spironium octonium, n. sp. (Pl. 49, fig. 7).
Cortical shell lentelliptical, its breadth (or transverse axis) surpassing considerably the length (or the principal axis). Surface thorny and somewhat hump-backed. The spiral wings of the transverse girdle have about the same breadth as the eight internal gates between them and the axial beams.
Dimensions.—Length of the cortical shell (or principal axis) 0.15, breadth (or transverse axis) 0.2; length of the medullary shell 0.05, breadth 0.04.
Habitat.—South Atlantic, west of Tristan da Cunha, Station 332, depth 2200 fathoms.
- ↑ Spironium = Shell with spiral structure; σπειρώνιον.
