96 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



calcareous plug was formed to fill up the opening through which the 

 axial canal had formerly passed. The tendency in the development 

 of the animal, so far as the base is concerned, was now toward an 

 elimination of the infrabasal circlet as composed of distinct elements. 

 This took place probably as a result of the consolidation of this 

 newly formed centrale with the surrounding infrabasals. The con- 

 solidation "irregularly became effective in some individuals and 

 ineffective in others," dependent, as explained above, upon the fact 

 that some individuals were more accelerated in development than 

 others. This consolidation of the proximal circlet is a tendency to 

 be observed among all crinoids, and especially is it to be noted in 

 free-swimming types such as, for instance, Agassizocrinus and some 

 species of Edriocrinus. In such recent forms as Antedon and Acti- 

 nometra the infrabasals are practically obsolete and fuse very early 

 with the proximale. Even the basals, which to all intents and pur- 

 poses have now become the proximal circlet, are affected by the 

 same tendency, fusing to form the so-called "rosette." It might 

 weU be that if given sufficient time the basals of Uintacrinus would 

 in turn be reduced and fused with the centrale. Indeed figure 9 

 seems to show that such a process had already begun. Here the 

 presence of apparently four basals suggests the fusion of two of the 

 plates, although it is obvious that this condition of the plates may 

 be purely abnormal. 



It is evident that Bather's arguments in favor of his classification 

 have yet to be refuted. Wliether the change from Form D to Form M 

 or vice versa took place by the intercalation of infrabasals on the one 

 hand or their elimination on the other is of little moment so far as 

 this particular question is concerned. If there has been an elimi- 

 nation of the infrabasals by fusion or resorption, the resultant form 

 still remains a dicyclic crinoid, or at most may be styled a "pseudo- 

 monocyclic" form. In case the infrabasals have made their appear- 

 ance in Form M it wiU be impossible to prove that they do not rep- 

 resent the nearly obsolete plates of an ancestral pseudomonocyclic 

 crinoid. In this case the plates that had come to assume an insig- 

 nificant part in the makeup of the crinoid were brought back to a 

 state of comparative prominence under the stimulus of changed 

 conditions of life. Against this hypothesis it may be argued that in 

 typical Form M absolutely no trace of infrabasals may be seen. This 

 objection carries little weight, however, as the infrabasals in the 

 ancestral form may well have been present only in the larval stage 

 and have become quite obsolete in the adults. Whatever the process, 

 then, all one has is a change either from pseudomonocyclic to dicyclic, 

 or from dicyclic to apparently monocyclic or pseudomonocyclic types. 



Probable derivation of Uintacrinus. — ^Among all known crinoids 

 Uintacrinus possibly stands as the form best adapted to meet the 



