1232 THE VOYAGE OF H.M.S. CHALLENGES. 



Subfamily 2. SETHOPERIDA, Haeckel, 1881, Prodromus, p. 433. 



Definition. T ripocyrtida with the basal mouth of the shell fenestrated (vel 

 Dicyrtida triradiata clausa). 



Genus 550. Sethopera, 1 Haeckel, 1881, Prodromus, p. 433. 



Definition. S ethoperida (vel Dicyrtida triradiata clausa), with three divergent 

 ribs enclosed in the latticed wall of the thorax. Cephalis with a horn. 



The genus Sethopera is probably the oldest form of the Sethoperida, and may 

 therefore be regarded as the common ancestral form of this subfamily. The thorax 

 exhibits three radial ribs, which are completely enclosed in its wall. Sethopera may be 

 derived either from Dictyophimus or from another of the Sethopilida (Lamprodiscus, 

 Clathrocanium) by development of a basal lattice-plate closing the mouth). There 

 are often wide holes or open spaces between the three radial ribs. 



1. Sethopera tricostata, n. sp. (PI. 97, fig. 11). 



Cephalis subspherical, spinulate, with a pyramidal horn of the same length. Length of the two 

 joints = 5 : 8, breadth = 5 : 8. Thorax also nearly spherical; in the upper half with three stout, 

 prismatic, convex, divergent ribs, and three large ovate holes between them; in the lower half with 

 numerous small and irregular pores, spinulate. 



Dimensions. Cephalis 0'05 diameter ; thorax 0'08 diameter. 



Habitat. Western Tropical Pacific, Station 225, depth 4475 fathoms. 



2. Sethopera Oceania, Haeckel. 



Lithopera Oceania, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 297, Taf. iv. fig. 21. 



Cephalis hemispherical, with a small bristle-shaped horn of half the length (broken oft' in 

 Ehrenberg's figure). Length of the two joints = 3:4, breadth = 3:4. Thorax nearly spherical ; 

 in the upper half with three divergent ribs, separated by some larger, irregular pores ; in the lower 

 half with numerous small, irregular, roundish pores. 



Dimensions. Cephalis - 03 long, 0'03 broad ; thorax 0'04 long, 0'04 broad. 



Habitat. North Atlantic, Station 354, surface ; Canary Islands and Azores. 



3. Sethopera pyrum, Haeckel. 



1 Lithopera pyrum, Ehrenberg, 1872, Abhandl. d. k. Akad.' d. Wiss. Berlin, p. 297. 



Cephalis hemispherical, with a small conical horn of half the length. Length of the two 

 joints = 3 : 10, breadth = 3 : 1. Thorax pear-shaped, with rather large, regular, quincuncial pores ; 



1 Sethopera = Sieve-pouch ; 



