22 



SVEN LOVEN, ECHINOLOGICA. 



entire, and in one or two cases a fourth \ery minute single 

 and mesial plate Ijecomes visible from nnder the limiting plate 

 of tlie corona. Thus, wliile the corona increases in circunife- 

 rence and the buccal membrane expands, the interradial plates 

 at its adoral termination, like the ambnlaci-als, are snbject to 

 gradual reabsorption and transformation. 



Buccal interradiuni of ('idaris papillnta. 



When this process is followed thi'()ngh snccessive stages 

 of growth, there are found 



in the Cidaris Thouarsi A(;. 



Diam. 11 mm. Coroiial pairs T) and <> 



» 12 » - »5 and G 



» IG ' »6 and 7 



20 G and 7 



22 7 



in the Cidaris papillata Leske. 



Diam. 27 mm. Coronal pairs 6 and 7 

 4G » » ■> 7 and <S 



55 , . . iS 



. 62 » y — -' 6 and 7 



and it becomes evident that the ventral interradial plates are 

 detached and transformed rather irregnlarly and at a råte 

 slower than that of the dorsal increase. They are given off 

 one after another, and form together a triangle filling a small 

 space left betAveen the expanded buccal ambnlacra; in tlieir 

 smallness and tenuity they strongly contrast with the broad 

 and solid still tixed coronal plates, from which they are deri- 

 ved, and the råte of their formation appears not to correspond 



