810 THE CRINOIDEA 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. 
