Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/78

48, Laube, sp.

Syn. Monostychia australis, Laube; Clypeaster folium, Dunc, nec 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 Laganum and Arachnoides in the Scutellidae, I have now come to the conclusion 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.

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

This common fossil species belongs to the group which Laube would place amongst his Monostychiæ, 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 ambulacra and interambulacral spaces. The generative system and the madreporiform body are at the apex of a blunt surface. Each ambulacrum 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.