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



6. Dictyastrum triactis, Ehrenberg. 



Didyastrum triactis, Ehrenberg, 1872, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 306. 



Arms rectilinear, four times as long as broad, with parallel edges, pointed at the distal end, with 

 a short terminal spine. Diameter of the circular central disk equal to double the breadth of the arms. 

 Dimensions. Eadius of each arm 0'2, breadth 0'04. 

 Habitat. Pacific, Philippine Sea, Station 206, depth 2100 fathoms. 



7. Dictyastrum aculeatum, n. sp. 



Arms lanceolate, three times as long as broad, twice as broad in the middle as at either end, 

 with thorny surface and numerous conical terminal spines, one very large in the radius. Central 

 disk triangular, about as broad as the arms. (Resembles Rhopalastrurn arcticum, PI. 43, fig. t>, 

 but differs by the equal angles and the triangular disk.) 



Dimensions. Eadius of each arm 0'2, breadth 0'06. 



Habitat. Western Tropical Pacific, Station 225, depth 4475 fathoms. 



Genus 228. Rhopalastrum, 1 Ehrenberg, 1847, Monatsber. d. k. preuss. 

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



Definition. P orodiscida with three simple, undivided, chambered arms, without 

 a patagium ; triangular shell bilateral, one odd arm opposite to the odd angle between 

 two paired arms. 



The genus Rhopalastrurn, founded by Ehrenberg (1847) with a very insufficient 

 diagnosis, is here retained for those Trigonastrida that agree in the generic characters 

 with the only species figured by him, viz., Rhopalastrum lagenosum (compare my 

 Monograph, 1862, p. 500). It comprises, therefore, such Euchitonida as agree with the 

 preceding Dictyastrum in the simple form of the three arms and the absence of a 

 patagium, but differ from it in the different size of the three angles, and often also in 

 the divergent form and size of the three arms ; one odd arm is opposite to the odd angle 

 between the two paired arms. 



Subgenus 1. Rhopalastrclla, Haeckel. 

 Definition. Arms with blunt ends, without terminal spines. 



1. Rhopalastrum truncatum, Haeckel. 



Bhopcdaatrum truncatum, Haeckel, 1862, Monogr. d. Radiol., p. 500, Taf. xxix. fig. 6. 



Distance of both paired arms about half as large as their distance from the odd arm. All three 

 arms nearly of the same form and size, very short and broad ; their breadth nearly equals that of 



1 Rlwpalastrum = Club-star ; f'eVatfion, oiarooa. 



