1258 THE VOYAGE OF H.M.S. CHALLENGER. 



3. Plectojyyramis dodecomma, n. sp. (PL 54, fig. 6). 



Shell smooth, slenderly pyramidal, with straight outlines. CephaHs conical, obtuse, with thirty-six 

 small square pores (in nine divergent longitudinal rows, each of four pores). Collar septum (be- 

 tween cephalis and thorax) with four distinct collar pores. Thorax pyramidal, with nine very stout 

 and straight angular radial beams, which are connected by ten to fifteen broad horizontal nine- 

 angled rings. Each of the large regular, square meshes so produced contains a very delicate fenestra- 

 tion, witli twelve regular, square pores, separated by two transverse and three longitudinal crossed 

 threads. A very regular and remarkable structure. 



Dimensions. — Ce^jhalis 0"03 long, 0'02 broad; thorax (with ten rings) 0'2 long, 0'15 broad. 



Huiitat. — Central Pacific, Station 271, depth 2425 fathoms. 



4. Plectopyramis (juadratomma, u. sp. 



Shell smooth, slenderly pyramidal. Cephahs large, ovate, hyaline, with few very small basal 

 pores and deep collar stricture. Thorax pyramidal, with straight outline, and nine very stout and 

 straight radial beams, which are connected by fifteen to twenty horizontal nine-angled rings. Each of 

 the large regular, square meshes so produced is filled up by a very delicate arachnoidal network, 

 with very numerous and small square pores (as in the similar Ci^iclopi/ramis infundihulum, PI. 54, 

 fig. 7, which differs in the loss of the cephalis). 



Dimensions. — Cephalis 0'05 long, 0-03 broad ; thorax (with fifteen rings) 0'32 long, 0-2 broad. 



Habitat. — North Pacific, Station 244, depth 2900 fathoms. 



5. Plectopyramis trapezomma, n. sp. (PL 54, fig. 8). 



Shell smooth, broadly pyramidal, with concave lateral outlines. Cephalis hemispherical, small, 

 with numerous small, circular pores. Thorax pyramidal, with nine stout, concavely curved radial 

 beams, which are connected by twelve to sixteen horizontal nine-angled rings. The large meshes so 

 produced are in the upper half square, in the lower half parallel trapezoid (in the fifteenth ring 

 three times as broad as long), and filled up by a secondary arachnoidal network, with very numerous 

 and small square pores. 



Dimensions.— Ge-\)\\?i\\s 0'015 long, 0-025 broad ; thorax (with fifteen rings) 0'22 long, 0'2 broad. 



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



6. Plectopyramis polygonomma, n. sp. 



Shell smooth, pyramidal, very similar to the preceding species ; differing from it in the irregular 

 shape of the delicate polygonal (not tetragonal) secondary network, filling up the large rectangular 

 meshes. The cephalis is larger, hyaline, pear-shaped, pointed, without pores, with a deep collar 

 stricture. The nine radial beams of the thorax are less concave. 



Dimensions. — Cephalis 0-04 long, 0-03 broad ; thorax (with fifteen rings) 0-3 long, 0-22 broad. 



Habitat. — South Pacific, Station 295, depth 1500 fathoms. 



