1298 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 574. Sethocephalus, 1 Haeckel, 1881, Prodromus, p. 430. 



Definition. S ethocorida (vel Dicyrtida eradiata aperta) with discoidal, flatly 

 expanded thorax. Cephalis large, without horn. 



'The genus Sethocephalus (formerly proposed under the name Platycryphalus) 

 contains some few and rare Sethocorida, distinguished by the very large cephalis, the 

 collar opening of which is wide open, not closed by cortinar beams, and surrounded by 

 a very flat, nearly discoidal thorax, like the brim of a hat. It is possible that this 

 peculiar genus has been derived from a Tricyrtid (TJieocalyptra ?) by loss of the 

 original cephalis, and that the apparent large cephalis is the original thorax. 



1. Sethocephalus eucecryphalus, n. sp. (PI. 56, fig. 13). 



Cephalis very large, campanulate-conical, with irregular, polygonal pores and thin bars. Thorax 

 short, scarcely broader than the cephalis, beyond the sharp collar stricture expanded like the brim of 

 a hat, with few rows of irregular, polygonal pores. (In the specimen figured the thorax was only 

 half as broad as in another specimen found afterwards.) 



Dimensions. Cephalis 012 long, 0'09 broad; thorax O02 long, 012 broad. 



Habitat. South Pacific, Station 285, surface. 



2. Sethocephalus platycryphalus, u. sp. 



Platycryphalus sethodiscus, Haeckel, 1881, Prodromus, p. 430. 



Cephalis large, hemispherical, with irregular square pores and thin bars. Thorax flatly conical, 

 three to four times as broad as the cephalis, beyond the collar stricture expanded nearly discoidally, 

 with a delicate network of small, subregular, hexagonal pores. (Similar in general form to 

 Scthophormis aurelia, PI. 55, fig. 3, but without any trace of collar beams or radial ribs.) 



Dimensions. Cephalis 0'04 long, O08 broad ; thorax 0'04 long, 0'3 broad. 



Habitat. South Pacific, Station 289, surface. 



Genus 575. Sethocyrtis, 2 n. gen. 



Definition. Sethocorida (vel Dicyrtida eradiata aperta) with ovate or 

 subcylindrical thorax, the mouth of which is constricted, simple, without prominent 

 hyaline peristome. Cephalis with an apical horn. 



1 Sethocephalus=S>ieve-hea<l ; aii6u, 



2 Sethocyrtis - Sieve-basket ; aq0u, x 



