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



Subgenus 2. Diplactinium, Haeckel. 

 Definition. — Ai"ms on the distal end armed with a radial terminal spine. 



2. Diplactura diploconus, n. sj). (PL 38, fig. 5). 



Amphiaetura diploconus, Haeckel, 1877, MS. et Atlas (pi. xxxviii. fig. 5). 



Phacoid shell three times as broad as the medullary shell, with eight jjores on its radius, 

 surrounded by a single chambered ring. Arms club-shaped, twice as long as the diameter of the 

 phacoid shell, at the distal end as broad as the latter, at the base only 4ialf as broad. Both poles 

 of the common axis of the arms are armed with a strong spindle-shaped terminal spine. 



Dimensions. — Diameter of the phacoid shell 09, of the medullary shell 003 ; length of the 

 arms (without terminal spines) OlT, basal breadth O'Oo, distal breadth 0'09. 



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



Genus 205. Amphiaetura,^ Haeckel, 1881, Prodromu.?, p. 458. 



Definition. — C occodiscida with t^vo opposite chambered arms on the margin 

 of the circular disk, connected by a spongy patagium. 



The genus Amphiaetura differs from the foregoing Diplaetura in the development 

 of a patagium between the arms, and therefore bears the same relation to it as Amphy- 

 menium in the Porodiscida does to Amphihraehium, or Spang ohrachium in the Spongo- 

 discida does to Spongolene. In this and in the following Discoidea provided with 

 a patagium, this connecting web constantly exhibits a different texture of its framework, 

 which is sometimes more regularly chambered, at other times more iiTegularly sjiongy. 



1. Amphiaetura amphibrachia, n. sp. (PL 38, figs. 3, 4). 



Phacoid shell three times as broad as the medullary shell, with eight pores on its radius. Arms 

 nearly equilateral triangular, twice as long as the diameter of the phacoid shell, at the truncated 

 distal end as broad as the latter, at the base only one-third as broad. Patagium a circular 

 lenticular disk, enveloping only the basal third of the arms, with three to four concentric circular 

 rings, divided into chambers by about forty radial beams, which are prolonged beyond the margin of 

 the patagium into radial spines. The vertical section (fig. 4) shows that the chambers of each 

 arm (eleven to twelve transverse rows in the radius) are disposed in two layers 



Dimensions. — Diameter of the phacoid shell 009, of the medullary shell 003 ; length of the 

 arms 018, basal breadth 0'03, distal breadth 0'09. 



Habitat. — Pacific, central area, Station 263, depth 2650 fathoms. 



' Amphiaetura =^tA\ai^di shell with tail on both sides ; ot^ipi', azTiV, ovna. 



