1244 THE VOYAGE OF H.M.S. CHALLENGER. 



2. Sethophormis medusa, n. sp. 



Cephalis very large, flat, cap-shaped, with irregular, delicate network of polygonal meshes. 

 Thorax flat, campanulate, nearly hemispherical, with delicate, irregular network of small polygonal 

 meshes. All bars very thin, thread-like. Four stout radial thoracic ribs, as in the preceding 

 species. Peristome denticulate. 



Dimensions. Cephalis 0'03 long, 0'09 broad; thorax 015 long, - 25 broad. 



Habitat. North Pacific, Station 244, surface. 



Subgenus 2. Pentaphormis, Haeckel, 1881, Prodromus, p. 432. 



Definition. Thorax with five radial ribs, one of the three primary ribs (-the odd 

 occipital rib) being simple, the two others (the paired lateral ribs) forked. 



3. Sethophormis pentalactis, n. sp. (PI. 56, fig. 5). 



Cephalis large, flat, hemispherical, with subregular, hexagonal pores. Thorax flat, campanulate, 

 nearly hemispherical ; also with subregular, hexagonal pores, and with five stout ribs. The three 

 primary ribs of the thorax are complete, and reach the peristome ; the two secondary ribs (as basal 

 branches of the two lateral ribs) are incomplete and absent in the distal half of the thorax. Peri- 

 stome denticulate, with an elegant marginal corona of small square pores and short conical spines. 



Dimensions. Cephalis 0'03 long, 0'06 broad ; thorax 01 long, 0'2 broad. 



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



4. Sethophormis asteriscus, n. sp. 



Cephalis large, flat, cap-shaped, with irregular polygonal pores. Thorax flat, campanulate, with 

 subregular, hexagonal pores, and five stout ribs. The marginal distance between the two anterior 

 (lateral) ribs is twice as great as the distance between every two other ribs (the postero-lateral ribs 

 being basal branches of the antero-lateral ribs). Peristome with a double corona of short, conical, 

 spines. 



Dimensions. Cephalis - 02 long, 0'04 broad; thorax 0'06 long, 017 broad. 



Habitat. Central Pacific, Station 272, depth 2600 fathoms. 



5. Sethophormis astrodiscus, n. sp. 



Cephalis small, flatly vaulted, cap-shaped, with irregular, polygonal pores. Thorax very flatly 

 vaulted, nearly discoidal, with subregular, hexagonal pores, and five stout ribs. The marginal dis- 

 tance between all five ribs is nearly equal. Peristome denticulate, with an elegant corona of small 

 square pores and short conical thorns. 



Dimensions. Cephalis O'OIS long, 0'035 broad ; thorax 0'05 long, 0'2 broad. 



Habitat. Tropical Atlantic, Station 348, depth 2450 fathoms. 



