SYSTEMATIC PART. 149 



When we discovered that the ventral surface of Taxocrinus, and probably 

 of all Ichthyocrinidse^ 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 Palaeozoic 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 Ichthyocrinidae 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 Paleozoic 

 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 Palseocrinoidea and Neocrinoidea as 

 natural groups, and proposed in place of them the Camerata, Inadunata, and 

 Articidata, 



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 Palaeozoic 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. PMla., 1888, p. 350, et seg. t Ibid., p. 345. 



