1202 THE VOYAGE OF H.M.S. CHALLENGER. 



2. Tripocyrtis tri^wdiscus, n. sp. 



Cephalis subspherical, with small, polygonal pores and a large, arborescent, vertical horn of thrice 

 the length, bearing numerous ramified branches. Thorax with three straight, widely divergent 

 ribs, of the same length as the horn, connected by a delicate, arachnoidal framework with irregular, 

 polygonal meshes. 



Dimensions. — Cephalis 0-04 long, 05 broad ; thorax 0-12 long, 018 broad. 



Habitat. — Central Pacific, Station 269, surface. 



3. Tripocyrtis plectaniscus, n. sp. (PI. 60, fig. 9). 



Cephalis subspherical, with small roundish pores, and a stout, three-sided pyramidal, denticulate 

 horn of the same length. Thorax with three little divergent and curved ribs, twice as long as the 

 horn, in the upper half with smaller, in the lower half with larger, polygonal, irregular meshes. 



Dimensions. — Cephalis 0'05 long, 0'06 broad ; thorax 01 long, 012 broad. 



Habitat. — Central Pacific, Station 264, surface. 



Genus 536. Sethopilium^ Haeckel, 1881, Prodromus, p. 431. 



Definition. — Sethopilida (vel Dicyrtida triradiata aperta) with three complete 

 thoracic ribs, which are prolonged into three solid divergent feet on the peristome. 

 Cephalis smooth, without a horn. 



The genus Sethopilium diflers from its probable ancestral form Dictyopkimus by the 

 complete loss of the cej^halic horn, and of the collar septum between both joints, 

 which are only separated by the slight external collar stricture. 



1. Sethopilium orthopus, n. sp. (PL 97, fig. 8). 



Shell smooth, with sharp collar stricture. Eelative length of the two joints— 5 : 4, breadth = 7:6. 

 Cephalis large, subspherical, with numerous regular circular pores, twice as broad as the bars. 

 Thorax with tliree stout, straight, widely divergent ribs, prolonged into three straight pointed feet 

 of the same length. Between every two ribs, in the centre of the collar stricture, a single large 

 triangular mesh, and beyond this three to six rows of smaller irregular meshes. 



Dimensions. — Cephalis 0'05 long, 0'07 broad ; tliorax 0-04 long, 0-06 broad. 



Habitat. — Central Pacific, Station 266, depth 2750 fathoms. 



2. Sethopilium cyrtoptis, n. sp. 



Shell smooth, with sharp collar stricture. Eelative length of the two joints = 4:5, breadth = 

 7 : 11. Cephalis hemispherical, with large irregular roundish pores, four times as broad as the bars. 



* SeihopiUwm = Bmai\ lenestrated hat ; aiiia, 'Kihinv. 



