2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



They first appeared in the Lower Jurassic, increased in niimher of 

 species throughout the Mesozoic, and reached their zenith in the 

 Eocene. Very few species are Hving today. 



In the course of this study, some interesting evolutionary trends 

 have been discovered, suggesting many hneages within the cas- 

 siduloids. Perhaps the most striking of these trends is the abrupt re- 

 duction from two pores to one pore in each ambulacral plate beyond 

 the petals, and the introduction of buccal pores. These changes oc- 

 curred in almost all cassiduloids in the Cenomanian. Likewise an 

 abrupt change in the structure of the apical system occurred at the end 

 of the Cretaceous, with most pre-Maestrichtian species having a 

 tetrabasal system and all Tertiary species having a monobasal system. 



\ 



Phyilodes 



Bourrelets 

 Peristome 



Petals 



Apical system 

 Periproct 



Fig. I. — An adapical and adoral view of a typical cassiduloid echinoid. The phyilodes ar€ 

 formed by the crowding of ambulacral pores near the peristome. The floscelle is the starlikt 

 structure formed by the phyilodes and the bourrelets. 



ACKNOWLEDGMENTS 



The National Science Foundation generously provided two grants 

 which made possible two trips to Europe to collect and to study in 

 museums in Paris, Basel, Geneva, Brussels, Liege, London, and Cam- 

 bridge. I am greatly indebted to Richard V. Melville, who accom- 

 panied me on a fossil-hunting trip in France and who was of great 

 assistance. General Collignon, Prof. A. Jeannet, Count Lecointre, and 

 Roger Brun very kindly accompanied us on part of this trip and 

 guided us to many echinoid localities. Max Meijer and Dr. G. Ubaghs 

 took me to several excellent echinoid localities in Belgium. 



Here in the U. S. National Museum, I am indebted to Dr. Richard 

 S. Boardman, who has critically read the manuscript and made many 



