90 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



come from ? There are but two possible ways to account for their 

 appearance. In the first place, the plates might have arisen as abso- 

 lutely new elements in a true monocycHc crinoid. This theory, as 

 hereafter explained, I consider absolutely untenable. In the second 

 place, if Vintacrinus were descended from a pseudomonocycHc form 

 the infrabasals wliich in the ancestral type were practically obsolete, 

 upon the assumption of an eleutherozoic existence, ndght have begun 

 to appear as important elements of the cup. It is obvious that if we 

 accept the theory of the intercalation of infrabasals we can not tell by 

 which of these two processes the plates were formed. 



I can not, however, in any way admit the possibiHty of the infra- 

 basals representing absolutely new elements in the cup. In the first 

 place, there is no logical reason for their existence. They add not at 

 all to the efficiency of the crinoid, and among the Crinoidea we see 

 httle or no aimless variation. The sole possible excuse for their intro- 

 duction would be the enlargement of the cup. The resultant amount 

 of enlargement is palpably insignificant and could quite as well have 

 been accomphshed by the enlargement of the centrale and proximal 

 plates, as in Marsupites. Again, the introduction of these new ele- 

 ments would necessarily result in a fundamental change in the nervous 

 system. The change from a dicycHc to a monocycHc type of nervous 

 system I consider quite witliin the range of possibiHty, but a reversal 

 of the process offers insuperable difficulties. The essential difference 

 between a dicycHc and a monocyclic crinoid can scarcely be empha- 

 sized too strongly. Certain it is that one form can only arise from 

 the other as the result of a long evolutionary process, and it is quite 

 inconceivable that, as would have to be the case in this instance, one 

 type should be a mere mutant of the other. 



Against the possibility of the enlargement of the minute infra- 

 basals of a pseudomonocycHc type one can not make so strong a 

 case. The uselessness of the proceeding is perhaps as good an argu- 

 ment in this case as in the preceding. One would certainly expect 

 that with the assumption of a free-swimming existence and the 

 formation of a centrale that the minute plates would fuse with the 

 new element. This would only conform to the general tendency 

 to be observed among nearly all crinoids toward the gradual elimina- 

 tion of the proximal circlet of plates as distinct elements. 



How are we to explain such types of bases as are shown in figures 

 5, 6, 7, and 8, Plate 9 ? Obviously figures 5 and 6 may represent 

 individuals in which three and four infrabasals have appeared, 

 respectively. It is not so easy, however, to account for figure 7. 

 Springer describes this specimen as having a double centrale and 

 one infrabasal. We are not told, however, by w^hat process two 

 centrales could be formed. If the centrale represents the rudiment 

 of the ancestral stem, could one individual have been the fortunate 



