REPORT ON THE RADIOL ARIA. 391 



2. Peripanarium cenocylindricum, n. sp. 



Internal cortical shell with four unequal clianibers, the two pro.xinial kidney-shaped, the two 

 distal conical, and somewhat smaller; pores irregular, polygonal-roundish, twice to five times as broad 

 as the bars. External cortical shell nearly spindle-shaped, in the equatorial zone inflated, conical, 

 tapering towards both poles, its pores very small, roundish, its surface covered with small thorns. 

 The shell of this species resembles very much Pcripanarius atradus (PI. 40, fig. 7), but differs by 

 the hollow cylindrical polar tubes, which arise from both poles of the inner cortical shell and attain 

 more than half the length of the main axis. They are open at their ends (broken away), about as 

 broad as the outer medullary shell, and perforated by irregular, roundish pores. 



BinunsioTis. — Main axis (without polar tubes) 0'28, greatest breadth (in the equator) 0'16; 

 pores of the inner cortical shell 0-004 to 0-01, of the outer 0-002 to 0-005; bars of the former 0-004, 

 of the latter 0-002 ; total length of the polar tubes 0-16, breadth of them 0-03. 



Hahitat. — Pacific, central area, Station 268, depth 2900 fathoms. 



Family XVII. Z y G A R t i d A, Haeckel (PL 40, figs. 10-13). 



Zijgartida, Haeckel, 1881, Prodromus, p. 462 (scnsu redrido). 



Definition. — Prunoidea with annulated cortical shell, the external shell being 

 divided by five or more parallel transverse constrictions into six or more chambers, en- 

 closing in the centre two internal concentric shells (medullary shells). Central capsule 

 cylindrical, commonly annulated (corresponding to the transverse annular constrictions 

 of the cortical shell). 



The family Z y g a r t i d a, the seventh and last of the Prunoidea, comprises, in 

 the sense here restricted, all those Prunoidea in which the cortical shell is annulated 

 and composed of six to twenty or more (at least six) camerse, lying one behind another 

 in the elongated main axis, and separated by five or more annular constrictions. When 

 I constituted this family in 1881 (loc. cit.), I had given to it a much wider extent, 

 embracing all those P r u n o i d e a which exliibit annular constrictions of the cortical 

 sheU ; as the number of these, in consequence of further researches, is much increased, 

 it seems now more convenient to restrict the family to the extent here given. 



No doubt the Zygartida must be derived from the Panartida by progressive growth 

 of the cortical shell in the main axis and accomj)anyiug increase of the number of its 

 chambers. AVhilst this number in the Panartida is constantly restricted to four, in 

 the Zygartida it amounts to ten, twenty, or more, and is in the lowest case six (PI. 40, 

 fig. 10). The maximum number is variable in the different sj^ecies, but of course also 

 different in the various degrees of individual development. Each Zygartid is at the 

 beginning of its growth a Cyphinid, later a Panartid. The number of the annular con- 

 strictions separating the chambers and lying in parallel transverse planes is at least 



