FROM THE WHITE CHALK. 



73 



" Table showing variation of form in the spines of Cidaris davigera — continued. 



Ambitus Spine, characteristic of tlie 

 general form of tlie whole of the 

 spines attached to any individual 

 test of C. clavigera, nat. size. 



Club-shaped, stem 



slightly tapering ; v. .m 

 common. 

 6. 



Club-shaped, stem (illji 

 cylindrical; com- "*'*' 

 mon. 

 7. 



Club-shaped, por- (■(iiiij 

 tion nearest the 



apex constricted ; 

 common. 

 8. 



Club-shaped, por- 

 tion nearest the 

 apex constricted ; 

 rather rare. 

 9. 



Constricted at about 

 half the length; U|.(jj 

 very rare. 

 10. 



Total 



length of 



longest 



ambitus 



spine. 



1-3 



1-0 



1-3 



1-0 



Diameter 



of spine at 



neck. 



Diameter 

 at greatest 

 thickness. 



Length of 

 neck before 



swelling 

 commences 



Form of apex. 



Hemi- 

 spherical. 



Hemi- 

 spherical. 



Hemi- 

 spherical. 



Hemi- 

 spherical. 



Acute. 



Diameter of test 

 at ambitus. 



About rO 



1-1 



11 



10 



