Dimensions.—Length of the first shell 0.02, second 0.05, third 0.09, fourth 0.16; breadth corresponding—(A) 0.015, (B) 0.035, (C) 0.065, (D) 0.12.
Habitat.—South Atlantic, west of Tristan da Cunha, Station 332, depth 2200 fathoms.
Genus 277. Larnacospongus,[1] n. gen.
Definition.—Larnacida with spongy lentelliptical cortical shell, without radial spines. Medullary shell double, Larnacilla-shaped.
The genus Larnacospongus differs from the nearly allied genera Larnacalpis and Larnacoma by the spongy texture of the lentelliptical cortical shell whilst the enclosed medullary shell in both genera is the same trizonal Larnacilla-shell. Therefore Larnacospongus (and the following nearly related Larnacostupa) can be derived directly by development of a spongy envelop either from Larnacilla and Larnacalpis, or from Trizonium and Amphipyle. But some species of these spongy genera appear to be derived rather from Tetrapyle or Pylonium, perhaps also from Cubotholus. Their phylogenetic origin may be explained in different ways.
1. Larnacospongus larnacillifer, n. sp.
Cortical shell lentelliptical, one and a half times as long as broad, with rough surface and rather loose spongy framework, directly enclosing a trizonal Larnacilla-shell of the same form, but of only one-third its size.
Dimensions.—Length of the whole shell 0.17, breadth 0.12; length of the medullary shell 0.06, breadth 0.04.
Habitat.—South Atlantic, east coast of Patagonia, Station 319, surface.
2. Larnacospongus tetrapylifer, n. sp.
Cortical shell lentelliptical, one and a third times as long as broad, with thorny surface; composed of an outer envelop of loose spongy framework and an inner lattice-shell with four kidney-shaped gates, like Tetrapyle; the latter encloses a trizonal medullary shell of one-fourth its size.
Dimensions.—Length of the whole shell 0.22, breadth 0.16; length of the medullary shell 0.045, breadth 0.035.
Habitat.—South Atlantic, west of Tristan da Cunha, Station 332, surface.
- ↑ Larnacospongus = Spongy chest; λάρναξ, σπόγγος.
