648 



MESSRS. RITCHIE AND McINTOSH ON 



[June 16, 



direction of their long axes, but, marked as the curves are, the 

 increase in the size of the plates is measurably of small moment. 

 The following comparative table indicates the differences in length 

 (i. e., in the direction of the long axis) and in depth (i. e., at right 

 angles to the long axis) of certain intei-ambulacral plates forming 

 a band, interrupted by the ambulacra, i-ound the skeleton. The 













CO 







CO 



CD 



X 





rO 



X 



X 



00 







CO 



o 



b 







rH 



« 



<^ 



ta 























00 











Jc- 



t^ 



CO 





e 



X 



X 



X 







GO 



o 



CO 







'"' 



C-5 











lO 



1 









00 









00 



X 



J>. 





rO 



X 



00 



X 







00 



b 



1> 









OJ 



<-l 



-* 















00 



lO 



CO 







CO 



CD 



lb 





e 



X 



X 



X 







r-t 



(M 



o 







(M 



(M 



(M 







lO 



to 



lO 





>o 



X 



X 



X 







05 



o 



o 







1-1 



(M 



N 



w 













(M 











lb 



lO 



lO 





e 



X 



X 



X 







05 



o 



r—^ 









(M 



« 







00 



^00 



io 







■^ 



■?? 



■^ 





<5 



X 



X 



X 





lO 



lO 



(M 







6 



6 



CD 







(M 



(M 



l-H 



(N 

















IC 











•^ 







lO 



lO 



X 





e 



X 



X 



O' 







a> 



o 



OS 







rH 



(N 









CD 











X 



CD 



lO 





►a 



lO 



X 



X 







05 



o 



o 









(M 



IM 



iH 























CD 











X 



CD 



CD 





e 



us 



X 



X 







00 



o 



o 









(M 



oi 





1 



1 : 



to 









I -^ 



H 









' (A 



S 



Tji 



CO 



(?j 





i-H 







i ^ 















•aiTTt 





6 ^ 



Qi ITS 



u .5 



,—, Qi 7^ 





missing ambulacrum is truncated opposite the thirteenth inter- 

 ambulacral plate, the plates being numbered from the peristome. 

 The thirteenth plate of each interambulacral area has, on this 

 account, been measured and along with it, for the sake of comparison, 

 the plate which bounds it on the proximal and on the distal side ; 



