DISCOVERY OF THE DISK OF ONYCHOCRINUS 481 



and Springer's interpretation of the conspicuous ring of plates was 

 substantially correct. As a matter of fact, there are two rings of 

 plates below the radials — i. e., both infrabasals and basals; but the 

 remarkable thing about it is that the infrabasals are developed to an 

 extent and in a manner unknown in any other Crinoid. In many 

 dicyclic forms the infrabasals lie within the ring of basals, abutting 

 against them by their lateral faces; and they are naturally subordi- 

 nate in size and position. But here they overlap the basals to such 

 an extent as to sometimes wholly conceal them, and not only them, 

 but also the radials, and even part of the first primibrachs (Plate VII, 

 Fig. 6). In some cases two or more of the basals are visible as mere 

 points (Plate VII, Figs. 3, 8) ; but usually only the posterior basal 

 projects (Plate VII, Figs. 2, 4). In one specimen no basal at all is 

 to be seen, and the large infrabasals appear externally to be directly 

 surrounded by the radials (Plate VII, Fig. 5). 



The three unequal infrabasals form a relatively enormous growth, 

 far exceeding in size the basals, and enveloping them somewhat after 

 the manner of the centrodorsal in Thiolliericrinus and other Coma- 

 tulae. The actual relation of the two sets of plates is show^n in speci- 

 mens like Fig. 6, Plate VII, where the infrabasals have been partly 

 removed, and the basals become plainly visible beneath them, five 

 in number. Thus the basal elements of the calyx are the same as in 

 the group generally — 3 IBB, and 5 BB — and the small infrabasal is 

 usually located, as it should be, under the right posterior ray. In 

 addition to the information furnished by Liljevall from the specimens 

 at Stockholm, I have several good specimens of this genus in my 

 own collection, which fully confirm the foregoing observations. 

 There remains no longer any doubt of its real structure, and the 

 genus must therefore be considered as representing a definite, though 

 extravagant and therefore short-lived, modification of the Crinoid 

 plan, in a direction not heretofore noted. 



This condition seems to be an exaggeration of that which obtains 

 in many of the Flexibilia, where the infrabasals have a tendency to 

 overlap the basals like a column plate, as in Forhesiocrinus and Taxo- 

 crinus. In these cases the union with the column seems to be stronger 

 than with the calyx; the infrabasals are frequently fused w^ith the 

 top columnal, and remain firmly soldered to it when the column is 



