49o 



HERRICK E. WILSON 



Various phases of the evolutional changes in the basal plates of 

 crinoids have been considered by both paleontologists and zoologists. 

 The relation of these plates to the column, their modification by 

 the anal plate, and their characteristic decrease in number' and 

 size with the passage of geologic time attracted attention long 

 before the importance of tegminal structure was realized, 1 and 

 some interesting though unsuccessful attempts have been made to 

 use basals as a foundation for classification. 2 Such artificial classi- 

 fications have been swept away, but the fact remains that the 

 changes of each plate and system of plates, as they have passed 

 through the varying stages of evolution, must have classificatory 

 value. When, where, and from what the monocyclic Crinoidea 

 originated are questions of great importance, yet not of special 

 import for the subject as herein discussed. The fact that mono- 

 cyclic crinoids have existed, that their basal plates exhibit a series 

 of remarkable changes, and that certain features in their phylogeny 

 throw much light upon the character of the changes and their 

 succession, is sufficient for present purposes. The process by 

 which these changes came about is the problem to be considered 

 here. 



In this paper no attempt at reclassification will be made, but 

 the changes exhibited by the basal plates will be reviewed, the 

 nature of these changes studied, arid suggestions arising from these 

 studies will be applied to certain theories of descent that have 

 arisen from similar studies made by others. 



ACKNOWLEDGMENTS 



The writer desires to express his sincere appreciation to Professor 

 Stuart Weller, of the University of Chicago; Mr. Frank Springer, 

 Mr. Austin Hobart Clark, Dr. E. O. Ulrich, and Dr. R. S. Bassler, 

 of the United States National Museum; and to Professor R. A. 

 Budington, Dr. Charles G. Rogers, and Dr. Maynard M. Metcalf, 

 of Oberlin College, for the material assistance given by them in 

 the use of specimens and literature, and for a broad view of the 



1 Ref. 38. (The reference figure is the number assigned to the work cited in the 

 Bibliography.) 



2 Ref. 2; ref. 22. 



