626 THE VOYAGE OF H.M.S. CHALLENGER. 



Dimensions. Length of the first shell O02, second 0'05, third O09, fourth 016; hreadth 

 corresponding (A) O'OIS, (B) 0'035, (C) O065, (D) 012. 



Habitat. South Atlantic, west of Tristan da Cunha, Station 332, depth 2200 fathoms. 



Genus 277. Larnacospongus? n. gen. 



Definition. L arnacida 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-sheU.. There- 

 fore Larnacospongus (and the following nearly related Larnacostupa) can be derived 

 directly by development of a spongy envelop either from Larnacilla and Larna- 

 calpis, 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-she}]. of the same form, but of only 

 one-third its size. 



Dimensions. Length of the whole shell 017, breadth 012 ; 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 016; length of the medullary shell 0'045, 

 breadth 0'035. 



Habitat. South Atlantic, west of Tristan da Cunha, Station 332, surface. 



1 Larnacospongus = Spongy chest ; ~ha,yiu.%,, a 



