SYSTEMATIC PART. 149 
When we discovered that the ventral surface of Taxocrinus, and probably 
of all Ichthyocrinide, is covered by a disk almost like that of recent Crinoids, 
and that it possesses an open mouth and open food grooves, it was instantly 
apparent to us that a division upon the line of Paleozoic and Neozoic 
Crinoids could no longer be maintained. The moment this truth was 
recognized, it was found to be reinforced by other considerations which were 
fully set forth by us at the time.* It might have seemed practicable to 
retain the two grand divisions upon the same characters as before, by trans- 
ferring the Ichthyocrinide to the division containing the recent Crinoids; 
but this would have made a change of the names unavoidable. Besides, the 
fact that those characters go back as far as the lowest Silurian was enough 
to suggest the gravest doubts whether the particular condition of the ventral 
covering was morphologically as important as we had supposed. 
About the same time we came into possession of specimens of the 
Camerate genus Platycrinus with orals almost perfectly symmetric, and the 
covering plates of the ambulacra most regularly arranged. Considering that 
in this same genus there are species in which the tegmen is composed of as 
heavy plates as in any Actinocrinus with subtegminal ambulacra, the con- 
clusion forced itself upon us that the plates of the tegmen in all these forms 
represent the same element, and that the most rigid “vault” of Palzeozoic 
Crinoids is but a modified disk. 
The change in our views was announced in our paper on “The Perisomic 
Plates,’ + in which we gave up the Paleocrinoidea and Neocrinoidea as 
natural groups, and proposed in place of them the Camerata, Inadunata, and 
Articulata. 
A separation of the older Crinoids into three divisions was attempted by 
us as early as 1877, and all that we have since learned, whether through our 
own observations or those of others, has tended to confirm their validity. 
Now that we have got rid of the imaginary line between Paleozoic and 
later Crinoids, we can better realize the importance of these groups, espe- 
cially since we find that they can be applied to all Crinoids, recent as well 
as fossil. 
We regard as the most important characters for dividing the Crinoids 
into orders : — 
1st, the condition of the arms — whether free above the radials, or partly incorporated 
into the calyx. 
* Proc. Acad. Nat. Sci. Phila., 1888, p. 350, ef seg. t Ibid., p. 345. 
