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



Genus 324. Zygacantha,^ J. Miiller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51. 



Definition. — As t rolonc h ida with simple, compressed, and two-edged radial 

 spines, without apophyses ; their transverse section is elliptical or rhomboidal. 



The genus Zygacantha comprised iu the original definition of J. Miiller only a 

 single species, Zygacantha furcata, distinguished from the other A c a n t h o m e t r a 

 by forked spines with two long parallel teeth. It seems now advisable to unite 

 in this genus all those Astrolonchida in which the simple spines are two-edged, 

 compressed, or leaf-shaped. The term Zygacantha may be conceived as the general 

 expression of the important fixct, that in all Icosacantha the twenty spines are opposite 

 in pairs. 



Subgenus 1. Zygacantharium, Haeckel. 



Definition. — Spines at the central base without leaf-cross and without hollow 

 pyramidal compartments, united by the opposed triangular faces of their pyramidal bases, 

 resting one upon another. 



1. Zygacantha lanceolata, Haeckel. 



AcantJiometra lanceolata, J. Miiller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. -18, 



Taf. xi. tig. 12. 

 Acmithometra lanceolata, Haeckel, 1862, Monogr. d. Radiol., p. 380. 



Spiues lanceolate, from the broader middle part equally thinned towards the two ends. Apex 

 simple. Base pyramidal, without leaf-cross. Each tiat lamellar spine exhibits an elevated middle 

 rib (like a lanceolate leaf), and is therefore compressed quadrangular. 



Dimetisions. — Length of the spiues 01 to 015, greatest breadth (in the width) 0-03 to 0'04. 



Habitat. — Mediterranean (Saint Tropez, French shore), J. Miiller ; North Atlantic (Canary 

 Islands), Haeckel, surface. 



2. Zygacantha costata, n. sp. 



Spines compressed, two-edged, linear, of nearly equal breadth in their whole length. Apex 

 truncate. Base pyramidal, without leaf-cross. Each Hat lamellar spine exhibits an elevated 

 middle rib, which in the distal half is cleft into two divergent rods ending iu the corners of 

 the truncated apex. (Similar to Zygacantha dicopa, but with broader free spines, which are not 

 grown together in the centre.) 



Dimensions. — Length of the spines 0"1 to 0'15, breadth 0'02. 



Habitat. — Central Pacific, Stations 265 to 274, surface. 



' Zi/r/(tcaB(/ia = Spines opposite in pairs ; i^vyi., axav^it. 



