48 P. M. DUNCAN ON THE ECHINODERMATA OF THE 



Arachnoides australis, Laube, sp. * 



Syn. Monostychia australis, Laube; Clypeaster folium, Dune, nee 

 Agass. ; Clypeaster, sp., Woods. 



Laube, in his interesting paper on the fossil Echinoderms of 

 the Murray Cliffs in South Australia (op. cit. p. 188), criticises 

 Woods, who termed a flat pentagonal fossil Clypeaster. This form 

 was seen by myself; and from its imperfect condition I was led to 

 believe that it was really a species common to Malta and some 

 other European Miocene localities. Mr. Etheridge, after examining 

 the specimen, concluded that Laube was correct in his criticism ; 

 for he determined that the form was not a Clypeaster, but a species of 

 a new genus, Monostychia, Laube. 



After carefully examining all the specimens I have been able to 

 obtain of this Clypeaster of Woods, and after carefully investigating 

 the value of the genus Monostychia in relation to Laganwm and 

 Arachnoides in the Scutellidae, I have now come to the conclu- 

 sion that they are not Clypeastroids, and that the proposed genus 

 is too closely allied to Arachnoides to be separated from it. Laube 

 distinguishes Arachnoides from his genus because the first has five, 

 and the latter only four genital pores ; but this is an error ; and he 

 makes the position of the periproct of generic importance in spite of 

 all the other great resemblances, this being an insufficient generic 

 differentiation. 



I have therefore placed the Clypeaster of Woods and myself, the 

 Monostychia australis of Laube, in the genus Arachnoides. 



Arachnoides elongattts, sp. nov. Plate III. fig. 8. 



This common fossil species belongs to the group which Laube 

 would place amongst his Monostychia^, but which, I think, fairly 

 comes within the genus Arachnoides. The test is longer than 

 broad, and is pentagonal, incised at the ambitus at the posterior 

 ambulacra and periproct, and faintly so at the ambitus of the other 

 ambulacra. The apical system is central. The test slopes very 

 gradually upwards from the ambitus for a little distance, and then 

 suddenly forms a sharp curve, whose sides are marked by the ambu- 

 lacra and interambulacral spaces. The generative system and the 

 madreporiform body are at the apex of a blunt surface. Each ambu- 

 lacrum is divided by a longitudinal groove, that of the anterior odd 

 one being the least developed ; and the ambulacral areas are rounded 

 and rise above the interradial spaces. The ambulacra are wide ; and 

 the poriferous zones form a curve on either side and externally, 

 but their inner edge is straight. The madreporiform body is large ; 

 the genital pores are large, and four in number ; the edge of the 

 ambitus is rather blunt ; and the actinal surface is nearly flat, except 

 near the peristome, where it suddenly sinks. 



The periproct is just under the margin. 



* This species is described in Laube's essay on the Fossil Echinida from 

 the Murray Cliffs in the Sitzungsberichte der kaiserlichen Akademie der Wis- 

 •enschaften, 1869, p. 190. 



