810 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



the structure of the ventral disk, in which the covering plates of the different 

 arms for quite a distance are suturally connected, so as to prevent motion. 

 It is further shown by the deep impressions upon the surface of the dorsal 

 cup, which evidently originated from constant pressure of the arms upon the 

 plates in the growing crinoid. 



