452 PEor. p. M. DirsrcAis' ojf the stritctijee of 



Fig. 6. A compound plate with a demi-plate of the same form (p. 429). 



7. Two compoimd plates of Fsev.dodiaderno. depressiim, var. (p. 430). 



8. Two compound plates at the ambitus of Plcsiodiadema Mickelini, Agass., 



showing the numerous primary plates (p. 431). 



9. A diagram of foiir compound plates of Pedi-na Smithi. a. Primary 



plates, h k c also primaries, a' & c' are demi-plates (p. 433). 



10. A single primary of Hernicidaris infermedia (p. 437). 



11. More or less deformed primaries (p. 438). 



12. The first compound plate from the union of two primaries, a third and 



untmited primary being beneath (p. 438). 



13. Two compoimd plates of Hernicidaris crenularis. The lower plate 



carries a great tubercle. Amb. iii. (Brit. Mus. No. 24122) (p. 439). 



14. Two compound plates of Hemirddaris crenularis (p. 440). 



15. Two compound plates of Hernicidaris intermedia : the adoral plate of 



the upper series has been formed into a demi-plate by growth- 

 pressure, and now forms a part of the lower plate (p. 442). 



16. A plate of Hernicidaris granulosa, showing affinities with Plesiodiadema 



(p. 442). 



17. The first triplet of a specimen of Heraicidaris (p. 443). 



18. The ambulacrum near the radial end of Diplopodia versijpora (p. 444). 



19. A compound plate of Liplopodia versipora, showing the persistence of 



the diplopodous arrangement (p. 4i5). 



20. A compound plate of the same species with a low primary plate (p. 446). 



21. Two plates of Diplopodia Malbosii, Desor. In the actinal there is the 



arrangement of Plcsiodiadema, and in the other that of the Liassic 

 Diadematidee (p. 447). 



22. A compound plate of Cyphosoma Konigi, Mant., with six plates showing 



the numerous sutural lines on the poriferous side and the two lines 

 towards the rertical suture (p. 447). 



23. A compound plate of the same species with fire plates and the sutural 



lines (p. 449). 



24. A compound plate from the same specimen showing the relics of the 



diplopodous arrangement, and indicating the alternation of large 

 and small plates and the direction of the sutures (p. 449). 



25. A diagram of the plates and sutures in a specimen of Cyphosoma 



Konigi, in the collection of the late Dr. Wright, F.E.S. (p. 450). 



All these figures are magnified and more or less diagrammatic copies from 

 nature, 



DisCTJssio:??. 



Mr. "W. Percy Sladex spoke of the importance of this communi- 

 cation, both, on account of its explaining points which were little 

 knownand for its bearing on classification, A. AgassizandLovenhave 

 cleared up some of the difScnlties in the classification of the regular 

 Echinoids, but probably this contribution to the question would 

 proTe of still more importance. He had himself had the opportunity 

 of following and confirming Dr. Duncan's observations, and expressed 

 the strongest conviction that the structures indicated would shortly 

 be shown to be of higher importance from a morphological point of 

 view than had hitherto been supposed. 



Prof. Seelet said that it seemed to him that this was a most 

 important contribution to zoological palEeontology, and its importance 

 would be more clearly seen as its bearings on classification and 

 evolution were traced out. It showed the eff'ects through genera- 

 tions of crushing upon the characters of the ambulacral plates. The 

 question then arises : As the plates are crushed together, forced 



