418 



PKOF. P. M. DUNCAN ON ECHXNOIDEA 



periproct in Arachnoides, and the variable notching of the test- 

 margin in Monostychia. It was evident that the position of the 

 periproct was variable in Arachnoides, and that it was found quite 

 marginal, as well as inframarginal, in Monostychia. M'Coy figured 

 the internal supporting structures of Monostychia in his ' Prodromus,' 

 and it agrees generally with the drawing by Laube, and there are 

 no structures to be seen near the margin of the test. He considers 

 Monostychia a good genus and retains it. Having had the oppor- 

 tunity of examining very much better specimens than those which 

 I studied formerly, and having seen a fortunate fracture of a speci- 

 men of Monostychia australis, Laube, I still find myself in the 

 difficulty of not being able to agree with my fellow-workers. My 

 reading of the nature of the internal part of the test does not agree 

 at all with that of my predecessors. But it is necessary that I should 

 state that, had I seen the specimen I now allude to, I should not 

 have placed the specimens in relation with Arachnoides. The new 

 specimens have enabled me to make out the distribution of the pores 

 beyond the petaloid parts of the ambulacra, and to describe the 

 nature of the plating of the actinal surface, and the nature of the 

 plates which enter the peristomial margin. It now appears to me 

 that the alliance is with Olypeaster, a notion which, so far as I am 

 concerned, dates back to 1864. 



In the figures published both by Laube and by Prof. M'Coy no 

 internal supporting structures are seen near the edge of the test. 

 This is unlike the structures of all Clypeasters ; but on examining 

 the worn edges of several of the tests in the National Collection, the 

 presence of upright and small needle-shaped pillars became evident, 

 and this Clypeastroid character became still more pronounced on 

 studying one of the specimens which had been fractured across. In 

 the best-preserved specimen the appearance of the structures which 

 occur between the upper and the actinal parts of the test within is 

 almost exactly identical with that presented by Olypeaster sub- 

 depressus, Gray, sp. The needle-shaped pillars with different 

 diameters are the same in structure as those represented in A. 

 Agassiz's ' Revision of the Echini,' 1874, pi. xiii. fig. 15. The flatness 

 of the test is quite equalled in small specimens of Olypeaster folium 

 from Corsica ; and there is no appreciable difference in this respect 

 between a rather elongated specimen of the Olypeaster and the 

 young specimens of Monostychia in the collection of the Society. 

 In both genera the ambulacra are grooved radially on the actinal 

 surface, but in the Australian forms the groove is continued defi- 

 nitely over the margin, and reaches up close to the actinal system ; 

 moreover, the edge of the test is notched at the ambulacral margins 

 at the place where the grooves are seen. The groove is not a simple 

 depression of the test along the median line of the ambulacrum, for 

 close to the peristome there is a slight median ridge, which is also 

 seen on the little projection which each groove makes into the 

 peristome and beyond the rest of its plates. In one specimen, pores 

 are seen on either side of the ridge, and connected with it by indis- 

 tinct grooves. Beyond the pores, and just within the interradial 



