BASAL PLATES IN CRINOIDEA CAMERATA 489 



PART III 



Evolution of the Base in Monocyclic Camerata 



1. Evolution of the Pentagonal Base 



2. Theories for the Evolution of the Base in Hexagonal Camerata 



a) Enlargement of the Posterior Basal 



b) Development of the Quadripartite Base 



c) Development of the Tripartite Base 



3. Evolution of the Base in the Hexacrinidae 



a) Evolution of the Tripartite Base 



b) Evolution of the Bipartite Base 



4. The Succession of Basal Changes in the Platycrinidae and the Hexa- 



crinidae 

 Summary of Conclusions 

 Bibliography 



PART I 



INTRODUCTION 



Most students of fossil crinoids have been interested in the 

 morphological results of evolution rather than in the morpho- 

 logical processes of evolution and their results, and the description 

 of extinct, fossil crinoids as well as of most living ones has been in 

 general a tabulation of those results. In many cases the tabulation 

 is incomplete, not only because of the rarity and incompleteness 

 of the specimens described, their poor preservation and sometimes 

 poorer preparation, but also because the obscure morphological 

 processes which have given rise to their characteristics are either 

 overlooked or misinterpreted. The greatest difficulty in estab- 

 lishing a natural classification lies in the fact that processes giving 

 rise to morphological characteristics must be known before a cor- 

 rect interpretation and classification can be made. From necessity 

 paleontologists are familiar with the obscurity of processes. The 

 ontogeny of the fossil crinoid is only partially revealed, and that 

 mostly in the phylogenetic succession, for the delicately constructed 

 embryos are ill-adapted for preservation, and comparatively few 

 immature individuals have been preserved. The complete em- 

 bryonic and larval development of modern crinoids is known only 

 in one highly specialized genus, Antedon; the larval stages, however, 

 are partially known in four other genera, Promacocrinus , Thaumato- 

 crinus, Comactinia, and Hathrometra. 



