NO, 3 CASSIDULOID ECHINOIDS — KIER I9 



chiis, the only important difference between them being that the 

 ambulacral plates of Arnaudaster and Parapygiis are single pored 

 beyond the petals and they have buccal pores. Because both of these 

 characters are advanced features and both Arnaudaster and Para- 

 pygiis occur later than Pygorhynchus, it is apparent that Pygorhyn- 

 chus is the ancestor. The stock that produced the Echinolampas- 

 Plesiolampas group appears to be derived from Parapygus. Both 

 Parapygus and Echinolampas have petals with narrow poriferous 

 zones that are unequal in length in the same petal, transverse, pen- 

 tagonal mouths, and strong but not pointed bourrelets. Plesiolampas 

 and Echinolampas are obviously evolved from the same stock inas- 

 much as they are nearly indistinguishable ; differing only in that in 

 Plesiolampas the periproct is longitudinal, whereas it is transverse 

 in Echinolampas. Conolampas is an offshoot from Echinolampas, 

 differing only in having straighter petals with narrower poriferous 

 zones. 



The Pliolampadidae are not homogeneous and may not represent 

 a natural or phylogenetic grouping. They are distinguished from the 

 Echinolampadidae in having petals with poriferous zones of equal 

 length and in usually lacking a naked zone in interambulacrum 5 

 adorally. They differ from the Faujasidae in having neither large 

 pointed bourrelets nor very broad phyllodes. They originated from 

 the Nucleolitidae, but it is not clear from what genus. The family can 

 be divided into two morphological and perhaps phylogenetic stocks : 

 one having open petals with very broad poriferous zones, including 

 Zuffardia, Termieria, Pliolampas, Pseudopygaidus, and Studeria, and 

 the other having closed petals with narrow poriferous zones, includ- 

 ing Gitolampas, Eurhodia, Ilarionia, Neocatopygus, Santeelampas, 

 Daradaster, and Kephrenia. Among those with broad poriferous 

 zones, Studeria greatly resembles Pliolampas and probably is de- 

 scended from it. Pseudopygaidus appears to be descended from 

 Termieria, both genera having broad petals, oval peristomes, trans- 

 verse and inframarginal periprocts, similar tuberculation, and phyl- 

 lodes with few pores. Pseudopygaidus differs in having no petal in 

 ambulacrum III. The relationships of the genera of the other stock 

 are not clear, although it appears that they may be offshoots from 

 a Gitolampas stock. 



The Clypeolampadidae include those genera, Vologesia and Clypeo- 

 lampas, with the test large and highly inflated, adoral surface flat, 

 open petals broad and long with wide poriferous zones, and phyllodes 

 with many pores. In shape and size they are very similar to Echino- 



