THE AMBULACRA OF FOSSIL ECHINOIBEA. 453 



downwards and x^artly absorbed by pressure, why do they group into 

 certain definite associations of plates and half-plates ? Prof. Seeley 

 had for many years had the opportunity of studying what had been 

 written upon this subject, and he thought that much light had been 

 thrown upon it by this paper. He believed the growth of the inter- 

 ambulacral plates had much influence on the form of the compound 

 plates of the ambuiacral areas. 



Dr. MuEiE considered that the paper was of great value from its 

 recognition of physiological facts, and its indication of their bearing 

 upon the theory of the evolution of organized forms. 



Mr. Etheridge called attention to the work of Dr. Wright on the 

 characters furnished by the ambulacra in the study of the regular 

 Echinoidea, and showed that Dr. Duncan's contribution had brought 

 into view the value of those investigations. The ambulacra were of 

 vast importance in classification, and they were most difiicult to 

 examine. Loven's work, too, was of very great importance. 



The Atjthoe, in reply, said that he had been much indebted to 

 Loven's investigations. Doubtless the action of growth-pressure which 

 he had referred to as " crushing," resulted in part in producing geo- 

 metrical figures, the interambulacra acting as " buffers" to the ambu- 

 iacral plates. The gradual formation of more complicated types 

 was very interesting. The increase in the number of pores above 

 the ambitus in the recent Diadematidse certainly meant increase in 

 power of respiration. 



Q. J. G. S. No. 163. 2 I 



