THE ECHINODERMATA 265 



the anus: they seem more comparable with the plates 

 of the anal pyramid in cystids. 



The radioles of Hemicidaris articulate with the tubercles 

 by a ball-and-socket joint. They are moved by muscles 

 in the skin, which lies outside the test. The tubercles are 

 distinguished according to size into primary, secondary, 

 and miliary. The primary tubercles consist of a boss 

 and a mamelon (Fig. 75, c) : in Hemicidaris the inner 



FlG. 76. CONULUS ALBOGALERDS, LESKE, EMSCHERIAN 

 (UPPER CHALK). 



a, Side view. (X^.) 6, Oral view. (Xj.) c, One interamb plate and 

 adjacent amb plates. (X2.) d, Apical disc. (Xa.) (After Wright.) 

 Sutures of amb plates omitted in a; of all plates in b; madreporite 

 dotted in d. 



part of the boss shows a ring of radial crenulations, and 

 the centre of the mamelon has a perforation ; so the 

 tubercles are described as crenulate and perforate. 



Conulus albogalerus (Fig. 76) is a common sea- 

 urchin of the White Chalk. The first obvious differences 

 from Hemicidaris are its more conical shape and much 

 smoother surface. Next we notice that the symmetry 

 is not strictly radial, but is bilateral, the base being a 

 pentagon of which one side is highly convex, while the 



