W. KEEPING ON THE PALEOZOIC ECHINI. 41 



genus Lepidesihes. These ambulacral areas exactly resemble the 

 figures of the American Melonites and Oligoporus; the plates are 

 covered with granules similar to those of the interambulacral areas. 

 Some of the interambulacral plates are distinctly overlapping ; but 

 in this character different plates vary very much, some of them 

 showing no trace of it. 



The thickness of the plates is great, being about equal to half the 

 exposed surface in the case of the interambulacral plates. 



Minute spines are profusely scattered over the whole surface of 

 the specimen. 



This fossil is, I believe, the first example recorded in Britain of 

 that group of Echini, with numerous ranges of ambulacral plates, 

 represented in America by the genera Melonites, Oligoporus, and 

 Lepidesihes. 



I hope soon to find time to give this specimen a more careful 

 examination, and to determine its affinities. 



EXPLANATION OF PLATE III. 



Fig. 1. Perischodomus biserialis (M'Coy); nat. size. The overlapped surface of 

 an interambulacral plate is exposed at x. 

 2. . Diagram to show the method of overlapping of the inter- 

 ambulacral plates from the central range outwards. 

 3. . Diagram to show the method of overlapping of the inter- 

 ambulacral plates from pole to pole. 



4. . Apical disk and portion of corona, enlarged 2 diameters. 



5. . Secondary spine, enlarged 4 diameters. 



6. JRhoechinus irregularis (Keeping), nat. size. 



7. . Portion of fig. 6, enlarged 2 diameters. 



8. . An interambulacral plate, enlarged. 



9. Palcschinus (?) intermedins (Keeping), nat. size. 



10. . Interambulacral^ plate, showing its bevelled edges, twice 



nat. size. 

 11. . Diagram to show the kind of overlapping of the inter- 

 ambulacral plates. 

 12. Palcechinus gigas (M'Coy). Professor M'Coy's figure. 

 13. . Corrected figure. 



14. Archcsocidaris TJrii (Fleming), nat. size. 



15. . A heptagonal interambulacral plate of the same. 



16. . Diagram to show the method of articulation of the inter- 

 ambulacral plates from pole to pole. 



17. . Diagram to show the method of articulation of the inter- 

 ambulacral plates from the central range outwards. 



18. A peculiar ambulacral plate from fig. 14, enlarged. 



Discussion. 



Mr. Etheridoe remarked that the genus established by the author 

 in this paper was of particular interest as representing an American 

 type, and it was of great importance in keeping up the correlation 

 of the geology of Europe and America. He then commented at 

 some length on the differences between the Palaeozoic and more 

 modern Echini. The type with flexible tests seemed never to have 

 become extinct. In conclusion he expressed his approval of Mr. 

 Keeping's proposed classification. 



