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



Synopsis of the Genera of Chiastolida. 



I. Subfamily 



Chiastolidina. 

 Sixteen diametral spines, derived by concrescence of thirty-two radial spines, . . 321. Chiastolw. 



II. Subfamily 



Acanthochiasmida. 

 Ten diametral spines, derived by concrescence of twenty radial spines, . . 322. Acanfhochiasma. 



Genus 321. Chiastolus, 1 n. gen. 



t 



Definition. C hiastolida with sixteen diametral spines, derived from thirty-two 

 radial spines opposite and grown together in pairs. 



The genus Chiastolus, hitherto known only by a single observed specimen, 

 comprises the Chiastolida with sixteen diametral spines, which are loosely crossed in 

 the centre of the body. Four of these spines are equatorial, four polar, and eight 

 tropical. We derive Chiastolus from Actinastrum by concrescence of every two opposite 

 spines in one axis of the body (compare above, pp. 729, 732). 



1. Chiastolus amphicopium, n. sp. (PI. 129, figs. 3, 3a, 3b). 



Spines in the central part cylindrical, spirally convoluted in a very peculiar manner, broadened 

 towards both ends, strongly compressed, two-edged ; the broadest parts are the two truncated 

 distal ends, five to seven times as broad as the thinnest central part, which is placed between 

 two spindle-shaped intumescences. These cochleary central parts of the sixteen spines seem to 

 be resting one upon another. Each spine (composed of two opposite equal radial spines) has nearly 

 the form of a double oar. The single observed specimen (preserved in glycerine) exhibited a 

 most regular disposition of the thirty-two spines (grown together in pairs in the centre). The 

 diameter of the dark non-transparent spherical central capsule equalled one-fifth to one-fourth of 

 the total length of the double spines. When the soft parts of the body were destroyed by sulphuric 

 acid, the sixteen single spines were suddenly dispersed. 



Dimensions. Length of the sixteen double spines O5, distal breadth (of the truncated ends) 

 0'05, central breadth O'Ol; diameter of the central capsule 0'12. 



Habitat. South-east Pacific (near Juan Fernandez), Station 297, surface. 



Genus 322. Acanthochiasma? Krohn, 1860, Monatsber. d. k. preuss. Akad. d. 



Wiss. Berlin, p. 810. 



Definition. C hiastolida with ten diametral spines, derived from twenty radial 

 spines opposite and grown together in pairs. 



1 Chiastolm=Vfiih crossed arms ; x/airroV, <*<>! 2 Acanthochiasma = Spine-cross; Hx.a.vSa., x,i*ff*- 



