REPORT ON THE RADIOLARIA. " 1257 



The genus Plectopyramis differs from the preceding Setho2')yramis, its ancestral form, 

 only in the peculiar double fenestration of the shell, the large primary meshes of which 

 are separated bj' strong bars, and filled up by a very delicate arachnoidal network, com- 

 posed of small pores and very thin threads between them. This double lattice-work 

 often exhibits a very remarkable regularity. As in the preceding genus, the number of 

 radial ribs is commonly six or nine, rarely twelve to twenty or more. 



Subgenus 1. Hexapleuris, Haeckel. 



Definition. — Pyramidal shell with six radial main beams (sometimes five or seven 

 in individual varieties). 



1. Plecto])yramis magnifica, Haeckel. 



Polycystina magnifii-a, Bury, 1862, Polycystins of Barbados, pi. xi. fig. 



Shell smooth, slender, pyramidal. Cephalis large, ovate, pointed, separated from the thorax 

 by a very deep collar stricture, with few irregular pores. Thorax with six strong and straight 

 radial beams, which are connected by about twenty horizontal, hexagonal rings. The large meshes 

 so produced are regidar, square, and filled up by an extremely delicate secondary arachnoidal net- 

 work, composed of regular square pores (as in PI. 54, figs. 7, 8). 



Divunsions. — Cephalis 0-06 long, 0-04 broad ; thorax (with twenty transverse rings) 0'4 long, 

 .0-12 broad. 



Habitat. — Fossil in Barbados. 



2. Plectopyramis hexapleura, n. sp. 



Shell thorny, slender, pyramidal. Cephalis large, ovate, blunt, with deep collar stricture. 

 Thorax with six strong and straight radial beams, which are connected by numerous (twenty to 

 thirty) interrupted transverse bars. The large meshes so produced are irregular, quadrangular, and 

 filled up by irregular, delicate, secondary network, with small polygonal pores. 



Dimensions. — Cephalis O'OS long, 0-03 broad ; thorax 0-3 long, 01 broad. 



Habitat. — Central Pacific, Station 265, depth 2900 fathoms. 



Subgenus 2. Enneapjleuris, Haeckel. 



Definition. — Pyramidal shell with nine radial main beams (sometimes eight or ten 

 in indi\ddual varieties). 



(ZOOL. CHALL. EXP.— PART XL. — 1886.) Er 158 



