90 Reports and Proceedings — Geological Society of London. 



the elevation of the land of Eastern North America and to an 

 elevation of North- Western Europe, which is supposed to have 

 occurred at the end of Pliocene times. These elevations would 

 intensify the glaciation caused by the difference of direction taken 

 by the Gulf Stream. 



2. " On the Affinities of the Echinothuridge, and on Pedinothuria 

 and Elikodiadema, two new Subgenera of Echinoidea." By J. W. 

 Gregory, D.Sc, F.G.S. 



The author summarizes and discusses the literature bearing upon 

 the Echinothuridas, and brings forward arguments to prove that the 

 family is a member of the order Diademoidea, and is derived from 

 the Pedinidge, members of which are found in earlier rocks than the 

 Corallian, which contains the oldest member of the Echinothuridee, 

 namely, Pelanecliinus. He maintains that the extreme flexibility and 

 loose articulation of the plates of the living genera Asthenosoma 

 and Phormosoma are due to the diminished calcification of the plates, 

 and that these recent genera are extremely specialized forms, and 

 not primitive — the apparently primitive features of the family being 

 secondly acquired, not primseval. 



A description is given of forms belonging to two new genera, 

 described as Pedinothuria — a connecting-link between the Pedinidas 

 and Echinothuridse ; and EWcodiadema — which has a flexible test, is 

 a modified form of Pseudodiadema, and has probably arisen from the 

 adoption of a deep-sea life resulting in diminished calcification of 

 the test. 



3. " On Ech'nocystis and Palceodiscus, two Silurian Genera of 

 Echinoidea." By J. W. Gregory, D.Sc, F.G.S. 



The author gives a history of the genera Echinocystis, Salter, and 

 PalcBodiscus, Wyv. Thoms., redescribes their structures, and dis- 

 cusses their affinities. He concludes that Echinocystis is an echinid 

 and not a cystid ; and that Palceodiscus is an echinid and not an 

 asterid. In order to prevent confusion, he suggests the name Scolo- 

 cystis for a true cystid which was described by Hall as Echinocystis, 

 some years after the latter name had been applied by Wyville 

 Thomson to the fossil now concluded to be an echinid ; and also 

 suggests the name of Discocystis for the form named Echinodiscus by 

 "Worthen and Midler. 



In discussing the affinities of Echinocystis, Wyv. Thoms., the two 

 latest diagnoses of the Cystoidea (those of Professor von Zittel and 

 Professor Haeckel) are considered, and it is contended that they do 

 not enable us to draw any sharp line of distinction between cystids 

 and echinids. 



It is shown that the masticatory apparatus of Palaodiscus and 

 Echinocystis explains the origin of that structure in gnathostomate 

 echinids ; and furthermore, it is suggested that Echinocystis renders 

 probable the homology of the so-called " calycinal plates " of the 

 Echinoidea with the plates of the valvular pyramid of the Cystoidea, 

 and not with the calyx-plates. 



I'he author gives synopses of the order Cystocidaroidea of Prof, 

 von Zittel, and of its two families, Palajodiscidse and Echinocystidse. 



