20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



lampas and might be considered ancestral except that their petals 

 have broad, equal poriferous zones, whereas these are narrow and 

 of unequal length in Echinolampas. They are distinguished from 

 the Pliolampadidae by their larger, much more inflated tests, longer 

 petals, phyllodes with more pores, and naked zone in interambula- 

 crum 5 adorally. Their origin among the Nucleolitidae is not clear, 

 but they may have sprung from a genus similar to Hypopygurus, as 

 they have very similar petals. 



The Archiacidae have two series of pores in their phyllodes in 

 each half -ambulacrum, and therefore probably originated from some 

 genus in the Nucleolitidae, but it is not clear from which genus. 

 The two genera in the family, Archiacia and Gentilia, are closely 

 related, both sharing several unusual characters including the dou- 

 bling of pores in ambulacrum III and the absence or great reduction 

 of a petal in this ambulacrum. Gentilia is more advanced, having 

 single pores in the ambulacra beyond the petals (except in the adapi- 

 cal portion of ambulacrum III), whereas Archiacia has double pores. 



Apatopygus, the only genus in the Apatopygidae, is probably de- 

 scended from Nucleolites. In both genera the petals are straight, 

 narrow, open, with narrow poriferous zones, the periproct is supra- 

 marginal, the test is of a similar shape, bourrelets slightly developed, 

 and the phyllodes have two series of pores in each half-ambulacrum. 

 Apatopygus occurs much later and, as would be expected, there are 

 single pores in the ambulacra beyond the petals, whereas in the earlier 

 Nucleolites there are double pores. The absence in Apatopygus of 

 buccal pores in a single-pored phyllode and the presence of "pyrinid" 

 plating in the ambulacra beyond the petals distinguishes this genus 

 from all the other cassiduloids and casts some doubt on its affinities. 



DISTRIBUTION IN TIME 



The relative abundance of the known cassiduloid species is shown 

 on chart 6. One of the most striking features of this distribution is 

 their great abundance in the Eocene and their subsequent decrease 

 in the upper Tertiary and Quaternary. Over 500 species have been 

 reported from the Tertiary, but there are only 16 species living today. 

 Part of this great Eocene expansion is more apparent than real. 

 Because most of the earlier workers did not distinguish the Paleocene 

 from the Eocene, some of their species may belong to the Paleocene. 



The great decline in species may have been caused by a cooling of 

 the seas and an increase in competition from other echinoids. Evi- 

 dently the cassiduloids are almost restricted to warm water, as shown 



