1238 THE VOYAGE OF H.M.S. CHALLENGER. 



1. Sethomelissa hymenoptera, n. sp. 



Cephalis subspherical, thorny, with a bunch of three divergent, larger apical spines. Thorax 

 ;pear-shaped, with three broad, triangular, latticed wings, the superior edge of which is a horizontal 

 straight, and stout spine, nearly as long as the thorax. Pores irregular, roundish. 



Dimensions. Cephalis O03 diameter; thorax Oil long, 0'08 broad. 



Habitat. Central Pacific, Station 271, depth 2425 fathoms. 



. 

 Genus 555. Tetrahedrina, 1 Haeckel, 1881, Prodromus, p. 433. 



Definition. Sethoperida (vel Dicyrtida tr-iradiata clausa) with a three -sided 

 pyramidal thorax, the three edges of which are prolonged into three divergent, solid, 

 terminal feet. Cephalis with a horn. 



The genus Tetrahedrina, and the two following nearly allied genera, are distin- 

 guished by the possession of three free, divergent, terminal feet, projecting over the 

 latticed mouth of the thorax. Tetrahedrina may be derived directly from Dictyo- 

 phimus, by development of a horizontal latticed mouth -plate from the middle part of the 

 feet, the lower part of which remains free. This genus, not noticed by Ehrenberg, is- 

 similar to his Lithochytris, and common in the Barbados deposits. 



1. Tetrahedrina pyramidalis, n. sp. 



Shell three-aided pyramidal, smooth, with slight collar stricture. Length of the two joints = 1 : 4,. 

 breadth = 1:4 Gephalis hemispherical, with small, sparsely disposed pores, and a pyramidal horn of 

 the same length. Thorax with subregular, circular pores and three sharp edges, prolonged into three 

 divergent pyramidal feet of the same length. 



Dimensions. Cephalis 0'03 diameter ; thorax 012 long, 012 broad. 



Habitat. Fossil in Barbados. 



2. Tetrahedrina pyriformis, n. sp: 



Shell pear-shaped, rough, with distinct collar stricture. Length of the two joints = 2:6, 

 breadth = 2:5. Cephalis subspherical, with a conical horn of the same length. Thorax with sub- 

 regular, circular pores, ovate, at the broad base with three pyramidal, widely divergent feet of the 

 same length. (Similar to Lyehnocanium tribulus, Ehreuberg, 1875, loc. tit., pi. vii. fig. 1, but with 

 the mouth closed by a lattice-plate.) 



Dimensions. Cephalis 0'04 diameter ; thorax 012 long, 01 broad. 



Habitat. Central Pacific, Station 265, depth 2900 fathoms. 



1 Tetrahedrina =:Small tetrahedron ; 



