THE T" A MILT AKBACIAD^. 37 



ment of the central primary towards tlie median line removes 

 every chance of the other plates being other than demi-plates. 



In the zone " 5 " the succession is very simple until the eleventh 

 plate, for the plate 1 is a small primary and 2 is a demi-plate, 

 because 3 is a large primary expanding towards the median line. 

 Plate 4 is a low primary associated with 5, a large primary, and 

 6, a smaller one. These three plates form a combination of small 

 primaries with an intercalated large one ; and a little more ex- 

 pansion of the last would have made them demi-plates, that is, it 

 would have cut them off from the median line. 7 is a small soli- 

 tary primary ; and then comes a compound plate made up of 8, a 

 small primary, 9, a large primary expanding towards the median 

 line and not shutting out no. 8, but occluding 10, which is a 

 demi-plate. Then follows a normal plate-combination of 11, a 

 demi, 12, a large primary extending to the median line, and 13, a 

 small demi. 



III. The Badial Plates. (Plate I. Figs. 4 and 5.) 



The radial plates are large and never reach to the periproct ; 

 they are broader than high, and the sides which are between the 

 Basals are usually curved, the convexity being outwards. The 

 sides which are free are curved, with the concavity outwards, and 

 the adoral side is slightly incurved, there being a projection in 

 the median line which may produce a double curvature. This 

 adoral part is broad and its edge is thick in well-preserved speci- 

 mens ; and the median projection, which resembles a long narrow 

 arched ridge with an adoral point, overhangs the centre of the 

 edge. There are two small optic pores which penetrate the ad- 

 oral margin between the internal and the external or back and 

 front edges of the margin, so that they appear as small points on 

 either side of, and below the end of, the projecting median ridge. 

 The pores are separated at their exit from the lower edge of the 

 plate by a delicate intermediate process, which is continuous with 

 the overhanging process already described. 



In some specimens the median ridge is made up by the coales- 

 cence, more or less perfect, of a series of granules. It sometimes 

 happens that the terminal part of the ridge has been worn and 

 weathered, and then the appearance is given of the presence of a 

 single large external pore ; but it is an illusion. 



The outer surface of the radial plates is beautifully ornamented 



