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



straight terminal spines. Patagium broad, incomplete, witli circular perimeter. Called in honour 

 of my friend Dr. John Murray. 



DimensioTis. — Eadius of the larger arm (including the spines) 0'24, of the smaller O'lG ; distance 

 of the terminal points of the former 0'18, of the latter 0'09 ; diameter of the patagium 0'2. 



Habitat. — North Atlantic, Faroe Channel, Gulf Stream, surface, John Murray . 



Genus 227. Dictyastrum,^ Ehrenberg, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. 



Berlin, p. 830. 



Definition. — P o r o d i s c i d a witli three simple, undivided, chambered arms, 

 without a patagium ; triangular shell regular, with three equal arms and three equal 

 angles. 



The genus Dictyastrum is the simplest form of the Trigonastrida, or of the 

 Porodiscida, in which the margin of the central disk is furnished with three chaml)erci:l 

 arms. In Dictyastrum these are quite simple and regular, without a patagium, 

 separated by equal angles, so that the whole shell represents a regular, equilateral 

 triangle, if we connect the distal points of the arms by lines. The genus Dictyastrum, 

 founded by Ehrenberg in 1860, differs from his Rhopalodictyum — after his own 

 diagnosis — only by an insignificant dift'erence in the form of the simple arms, which 

 is scarcely a specific character. I therefore apply this name here iu the above amended 

 sense, seeing that the only figured species of Ehrenberg [Dictyastrum angulatum) 

 occurs in two difi"erent, but externally veiy similar forms : one of these is a true 

 Porodiscid (Dictyastrum) with two porous covering-plates and concentric rings ; the 

 other is a true Spongodiscid (Rhoimlodictyum) with quite spongy, irregular network, 

 and is probably identical with the Rhopalodictyum truncatum of Ehrenberg. 



Subgenus 1. Dictyastrella, Haeckel. 

 Definition. — Arms with blunt ends, without terminal spines. 



1. Dictyastrum angulatum, Ehrenberg. 



Dictyastrum angulatum, Ehrenherg, 1872, Abhantll. il. k. Akiul. d. Wiss. Berlin, p. 289, 

 Taf. viii. fig. 18. 



Arms nearly square, with straight edges, towards the truncated end a little broader, about 

 the same diameter as the triangular central disk. The figure of Ehrenberg seems to re^jresent a 

 Spongodiscid (Ehopalodictyum angulattmi), but in the same locality (Philipj)ine Sea) occurs also 

 a true Dictyastrum of cj^uite the same form, Ijut with three to four concentric rings of the central 

 disk, and with jointed arms. 



' Z)ici2/(7s/nini = Reticulated star ; o/kti/ov, uitiiov. 



