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S04 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
differ essentially in other respects. In the Reteocrinids these pieces are 
introduced along the sides of the radials and the lower brachials, and between | 
the primary interbrachials, but in the Acrocrinide 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 ox 
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. 
-i 
Fig. 21. Acrocrinus. 
6—hasals; R—radials; Y=costals; x—special anal plate. All the other plates are supplementary pieces. 
The Acrocrinidz are undoubtedly closely related to the Hexacrinide ; in 
fact the introduction of a narrow belt of supplementary pieces between the 
basals and radials would be sufficient to transform any Dichocrinus into an 
Acrocrinus. 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 interbrachials 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 
