134 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
The symmetry of the Crinoids, as a rule, is bilateral, and the anal area 
occupies the median axis. The asymmetry which occurs in many Fistulata, 
and in certain Ichthyocrinide, is caused by irregularities in the radials. 
Wherever these attain a regular form, the plate x takes its median position, 
and the plates of the ventral tube are arranged on a strictly bilateral plan. 
Whether the symmetrical calyx, as represented in the Silurian and later 
Cyathocrinidee, was evolved from the asymmetrical form, we are unable to 
ascertain. It may be that the two had a common symmetrical ancestor, or 
that all these Crinoids were primitively asymmetrical, and that the lower 
section of the posterior radial became early resorbed in some cases. Against 
the former theory it may be said that in the Lower Silurian Fistulata, so far 
as we know without exception, the right posterior radial is compound, and 
that the symmetrical form occurs with the other in the same families; against 
the latter, that the symmetrical form is already well represented in the 
Upper Silurian. 
We have made no reference here to the Calceocrinidxe and Catillocrinide, 
as we have not at present the material to study the older forms; but we feel 
quite certain that their structure in this respect shows no material departure 
from that of the older Fistulata. 
The anus of the Crinoids is located in the disk, and is either central, sub- 
central, or marginal, —in the latter case sometimes coming down to the 
arm region. In some species there is merely a simple opening passing out 
directly through the disk ; others have a tube with an opening at the distal 
end or along the side. The size of the tube is quite variable. In some 
genera it rises to a height of several inches beyond the arms; while in 
others it is less than half their length. The tube is composed of heavy, 
generally nodose, wedgeform pieces, admitting but little mobility in the 
structure. When there is no tube, the anus is generally situated within the 
centre of a wart-like inflation, composed of very minute pieces, which 
possibly were movable, and could be drawn in by the animal, like those 
in the “ proboscis” of recent Crinoids, so as to open or close the aperture. 
There has been some difference of opinion whether or not species with 
an anal tube should be separated generically from those with a simple 
opening. Considering the slight differences upon which many genera have 
been founded, it would seem that the tubular structure ought to be of 
sufficient importance to justify a separation; but considering that various 
groups, after being carefully restricted with reference to all other characters, 
