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



principal arm with twelve joints, one and a half times as long as the anterior (with nine joints) and 

 twice as long as the two lateral arms (each with six joints). The form and structure of the arms 

 in this species are nearly the same as in Tesserastrum straussi (PL 45, fig. 8) ; but the arms are 

 broader in the middle, and are not connected by a patagium. 



Dimensions. Radius of the principal posterior arm 0.'2, of the anterior 0'15, of each lateral 

 arm Ol ; greatest breadth (in the width) 001, basal breadth 0'03. 



Habitat. North Atlantic, Faeroe Channel, Gulf Stream, surface, John Murray. 



Genus 235. Histiastrum, 1 Ehrenberg, 1847, Monatsber. d. k. preuss. 

 Akad. d. "Wiss. Berlin, p. 54. 



Definition. P orodiseida with four simple, undivided, chambered arms, 

 connected by a patagium ; square shell a regular cross, with four equal arms and four 

 right angles between them. 



The genus Histiastrum, quite insufficiently characterised by Ehrenberg (1847), 

 was afterwards (1875) illustrated by the figures of two different fossil species. One of 

 these, Histiastrum ternarium, with three arms, belongs to Hymeniastrum ; the other, 

 Histiastrum quaternarium, is here retained as the true, typical representative species 

 of the genus. It differs from its ancestral form, Stauralastrum, by the possession of a 

 patagium, from Tessarastrum by the regular square form of the shell. 



Subgenus 1. Histiastrella, Haeckel. 

 Definition. Distal ends of the arms blunt, without terminal spines. 



1. Histiastrum quadrigatum, n. sp. (PI. 46, fig. 3). 



Arms at their distal end nearly as broad as long, and four times as broad as at their narrow base ; 

 their lateral edges concave, their terminal edge convex, without spines. Each arm is divided by 

 seven to eight convex transverse septa into eight to nine simple, broad chambers. Central disk 

 with three to four rings, about as broad as the fifth chamber. Patagium complete, connecting all 

 the lateral edges of the arms. 



Dimensions. Radius of each arm 015, basal breadth - 03, terminal breadth 012. 



Habitat. Equatorial Atlantic, Station 347, surface. 



2. Histiastrum excisum, n. sp. 



Arms four tunes as long as broad at their base, and twice as broad at their rounded blunt distal 

 end as at their base ; their lateral edges rectilinear, divergent. Central disk with three to four rings, 



1 Histiastrum = Star with enveloping tissue ; ianou, 



