BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 21 



And when tlie weight of the plate and that of the spine 

 a 8 are severally assumed as 1, the råte of growth is: 



a 8 b 7 (i 7 b 6 a 6 



Plate: 1. 3,oi(;. 4,93. 5.53. 5,9. 



Spine: 1. 49. 5291. 9423. 12640. 



It is seen that the increase is immensely in favour of the 

 spines. 



While the dorsal and middle part of the coronal interra- 

 dium is tlie seat of this vigorous growth, the reverse is the 

 oase in its ventral region. In any adult speoimen of Cidaris 

 it is readily seen that the scrobicnles of the interradial corona, 

 dorsally all but circular, downwards beeome more and more 

 Dval transversely, while their rings of verrnculse, which at 

 flrst keep distinct above and below, are seen to approximate, 

 then low^er down to intermingle, to beeome defeetive, and at 

 last to vanish, the scrobicnles nearest to the peristome rnn- 

 ning into one another. This is the eiFect of the pressnre from 

 above combined with the loss of snbstance going on in the 

 ventral parts. The plates near tlie stoma are seen to dimi- 

 nish; little by little their snperficial extent consumes from 

 reabsorption, they are cnt right oft' adorally by concentric 

 abrasion and what remains melts away. In part their calca- 

 reons substance consolidates again in the Ijnccal menibrane and 

 reappears there as single or double scales, niinnte, thin and flat. 

 .■^lightly imbricated, withont even a trace of a verrnca, but 

 l)earing a row of verrncnl{€ at their straight adoral margin. 

 This process is analogous to that observed in the ambulacra. 

 Imt here it takes place less regularly and more sparingl}' 

 and begins somewhat låter; the adoral end of each triangulär 

 set remains behind that of the ambulaorals. The huge pri- 

 mary, raesial and solitary plate of the interradium in Gonio- 

 cidaris, fig. .9, is the first to be reabsorbed and reniodelled. 

 and after it come successiveh^ the following binar}^ plates. 

 If now tliese same parts are examined in a specimen of Ci- 

 daris papillata 42 mm. in diameter' this buccal series consists 

 of six plates, all simple or nearly so. In a young Cidaris 

 Thouarsi Ag., of 11 mm., three minnte interradial plates are 

 found in the buccal membrane, the first single, the second 

 mostly also single and entire, but in one or two of the inter- 

 radia divided into a pair, the third everywhere single and 



' See the woodcut on next jjage. 



■^ 



