Herbert L. HawJcins — Studies on the Echinoidea. 489 



lithologically very similar sedimentary series of the South. Orkneys 

 is definitely known to be of Lower Palaeozoic age (Silurian) from the 

 evidence of graptolites.^ It is a fact suggestive of close geological 

 relationship between South Georgia and the South Orkneys that 

 a small pebble of typical spilite was found by me in a series of rocks 

 collected by the Scotia Expedition from Coronation Island in the 

 South Orkneys.' 



According to Dewey and Flett spilites are the characteristic 

 volcanic rocks of districts that have been undergoing long-continued 

 subsidence.^ This relation receives further exemplification from 

 South. Georgia, where the sedimentary series is of great thickness 

 and continuity, and lithologically is of shallow- water origin. 

 Benson has shown that spilites are not necessarily indicative of 

 deep-water conditions as supposed by Steinmann and others.* 

 Lastly, attention may be drawn to the fact that the South. Georgia 

 sediments are radiolarian, another character often exhibited by the 

 sedimentary rocks associated with a spilitic suite. 



The existence of a spilitic suite in South Georgia and the South 

 Orkneys has an important bearing upon the tectonic relations of 

 these islands. The balance of evidence may now be said to tip 

 definitely against Suess's interpretation of their structure. He 

 believed that they formed part of a great eastwardly-directed loop, 

 homologous with the Antilles, connecting the Patagonian Andes 

 with the mountain ranges of Graham Land. The continued absence 

 of typical Andean volcanic rocks in spite of repeated collection of the 

 rocks of South Georgia, as well as the existence of a spilitic series, 

 favours -^the interpretation that South Georgia and the South 

 Orkneys are remnants of an ancient continental land which once 

 occupied the South Atlantic.^ 



III. — Morphological Studies on the Echinoidea Holectypoida 



AND THEIK ALLIES. 



By HeebePvT L. Hawkins, M.Sc, F.G.S., Lecturer in Geology, University 

 College, Eeading. 



"VIII. On Pygastbides, Loven, a Problematical Holectypoid. 



(PLATE XVII.) 



1. Introduction. 



IT may be thought that some apology is due to the readers of this 

 Magazine on the ground of the largely zoological bearing of this 

 paper. But in the opinion of the writer no such apology is necessary. 

 Zoology is palseontology brought up to date, and ontogeny is but 

 a compressed and individualistic type of phylogeny ; so that in 

 spirit, though not in matter, this paper is no less appropriate for 



1 J. H. Pirie, Proc. Eoy. Soc. Edin., vol. xxv, pp. 463-70, 1905. 

 - Op. cit. supra, p. 833. 



2 Op. cit., p. 242. 



•• W. N. Benson, " Spilite Lavas and Radiolarian Rocks of New South 

 Wales " : Geol. Mag., dec. V, Vol. X, pp. 17-21, 1913. 



^ J. W. Gregory, " The Geological Relations and some Fossils of South 

 Georgia" : Trans. Roy. Soc. Edin., vol. 1, pt. iv, pp. 817-22, 1915. 



