804 



THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



differ essentially in other respects. In the Eeteocrinidfe these pieces are 

 introduced along the sides of the radials and the lower brachials, and between 

 the primary iuterbrachials, but in the Acrocrinidas below the radials and 

 between them and the basals. In the one group the basals are in contact 

 with the radials, and the radials are separated all around, while in the other 

 the radials are in contact except at the anal side, but are widely separated 

 from the basals by a new set of plates, for which the term " subradials," if it 

 had not been otherwise used, would be appropriate. 





rig. 21. Acrocrimis. 

 b = basals ; R = radials ; /= eostals ; x = special anal plate. All tlie other plates are supplemeutary pieces. 



The Acrocrinidse are undoubtedly closely related to the Hexacrinidce ; in 

 fact the introduction of a narrow belt of supplementary pieces between the 

 basals and radials would be sufficient to transform any Dicliocrinus into an 

 Acrocrimis. In both genera there are two equal basals, the radials of both 

 enclose an anal plate, in both of them the arms are sometimes pendent, and 

 neither one has iuterbrachials entirely within the dorsal cup. Considering 

 that the number of these supplementary pieces increases with the growth of 

 the crinoid, as is shown conclusively from the specimens, it seems to us more 



