1290 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 572. Sethoconns,^ Haeckel, 1881, Prodromus, p. 430. 



Definition. — S e t h o c o r i d a (vel Dicyrtida eradiata aperta) with conical or 

 campanulate, gradually dilated thorax and wide open mouth. Cephalis with one or 

 more horns. 



The genus Sethoconus is the oldest and simplest form of the Sethocorida, or of those 

 Dicyrtida in which the simple thorax bears no apophyses and has the mouth open and not 

 closed by a basal lattice-plate. Probaljly all those eradiate Sethocorida have been derived 

 either from triradiate Sethopilida {Lychnocanium) , or from multiradiate Sethophormida 

 {Sethophormis, Anthocyrtis 1) by loss of the radial ribs and feet. In Sethoconus the thorax 

 is more or less conical, sometimes more campanulate, commonly smooth, in some 

 species covered with spines. The cephalis is usually small, with rudimentary collar 

 septum, at other times large, with distinct cortinar septum. Correspondingly, the genus 

 may be divided into different groups, which here are enumerated as subgenera. 



Subgenus 1. Conarachnium, Haeckel, 1881, Prodromus, p. 430 

 ( = Ceratocyrtis, Biitschli), 1882, loc. cit., p. 536. 



Definition, — Cephalis relatively large, with distinct collar septum and numerous 

 pores. Thorax smooth. 



1. Sethoconus trochus, Haeckel. 



Eucyrtidium trochus, Ehrenberg, 1872, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 293, 



Taf. vii. fig. 17. 

 Conarachniura troclms, Haecksl, 1881, Prodromus, p. 4.30. 



Cephalis ovate, relatively large, with a pyramidal horn of the same length, and three pairs of 

 large opposite pores, on each side of a vertical septum. Thorax wide, conical, about as long as 

 broad, with nearly straight outlines ; its pores regular, circular, hexagonaUy framed, of the same 

 size as the cephalic pores. 



Dimensions. — Cephalis O'Oo long, 0"02 broad; thorax 0'06 long, 0'06 broad. 

 Habitat. — Tropical Pacific, Stations 200 to 281, surface. 



2. Sethoconus cucullaris, Haeckel. 



Coniutella nicullaris, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 68, Taf. ii. 



fig. 7. 

 Ceratocyriis ciwuUaris, Biitschli, 1882, Zeitschr. f. wiss. ZooL, Bd. xsxvi. p. 536, Taf. xxxiii. 



fig. 36, a. h. 



Cephalis subspherieal, thorny, relatively large, with a conical horn of the same length, and small 

 circular pores. Thorax wide, conical, enveloping the lower hidden half of the cephalis, about as 



' Sethoconus = Sieve-cone ; ai^a, kui/o:. 



