NO. 3 CASSIDULOID ECHINOIDS — KIER 1 5 



SUMMARY OF EVOLUTIONARY TRENDS 



From a study of the cassiduloids the following evolutionary trends 

 are apparent: 



1. The ambulacral plates beyond the petals have double pores in 

 all the pre-Cenomanian species, but single pores in most of the Upper 

 Cretaceous and all the Tertiary species. 



2. Buccal pores are introduced at the same time that the double 

 pores are reduced to single pores. 



3. The phyllodes become shorter and wider, have fewer pores, and 

 become single pored. 



4. There is an increase in the area of test below the petals. 



5. The outer poriferous zone in the petals is narrower in the later 

 species. 



6. The apical system in the earlier species is tetrabasal, often with 

 complementary and catenal plates. The complementary and catenal 

 plates are lost by the end of the Jurassic, and at the end of the Cre- 

 taceous the system becomes monobasal. 



7. The bourrelets reach their zenith of development in the Mae- 

 strichtian. 



8. The periproct is typically supramarginal in the more primitive 

 species. 



9. The tubercles become larger adorally, and in the later species a 

 naked area may be present adorally in interambulacrum 5. 



10. The shape of the test changes from circular to elongate. 



PHYLOGENY 



Using these evolutionary trends as the criteria, it is possible to 

 suggest the phylogeny of the cassiduloid genera (chart 5). 



The Galeropygidae are the earliest of all the cassiduloids, and the 

 most primitive in that they have a supramarginal periproct in contact 

 with the apical system, very slightly developed petals, a low, wide 

 test, and long, narrow phyllodes. Within the family, Ilyhoclypiis, 

 which is more advanced in having an elongate test, appears descended 

 from Galeropygus. 



The Clypeidae are descended from the Galeropygidae or from a 

 close ancestor of the Galeropygidae. They are more advanced as 

 shown by their well-developed petals and usually more marginal or 

 inframarginal periproct. Within the family, Pseudopygurus, Pyguriis, 

 and Clypeus are closely related, all three having well-developed 

 petals, a low test, and in the earlier species narrow phyllodes with 



