124 ANNULOIDA. 



The dorsal surface of the cup or " calyx " is composed of a 

 number of calcareous plates accurately fitted together. The 

 number and arrangement of these vary much in different 



Fig. 78. PentacrJHUS /asciciftosus, showing the form of the column, and one of the 

 facets of a joint The central figure shows the arms, and the summit of the column 



with side-arms. 



genera, and it will be sufficient to indicate here their general 

 disposition. Reposing directly upon the summit of the column 



