NO. 3 CASSIDULOID ECHINOIDS — KIER VJ 



more elongate test and more marginal periproct. Oolopycjus appears 

 to be a more advanced offshoot of Catopygns. It is similar to Cato- 

 pygus in having a small, elongate, moderately inflated test, narrow, 

 equal petals, marginal periproct, and longer than v^ide peristome. 

 It is more advanced in having single pores in its ambulacral plates 

 beyond the petals, and in having buccal pores. The position of 

 Hemicara is not clear. It resembles Catopygiis in its floscelle and 

 slightly developed petals, but has an inframarginal periproct. Pygau- 

 lus appears to be an offshoot from Catopygiis. Both genera have a 

 cylindrical test, and narrow, straight, open, petals extending to the 

 margin. However, the peristome in Pygaulus is oblique, and it is 

 not closely allied with Catopygus. Both Pygorhynchus and Plagio- 

 chasma are similar to Pygaulus. All three genera have the peristome 

 oblique (although some species of Pygorhynchus have it regular), 

 open petals with narrow poriferous zones, similar phyllodes, and 

 longitudinal periproct. They differ only in the location of the peri- 

 proct and shape of test. Pygopistes is apparently a direct offshoot 

 from Plagiochasma in having the same petal arrangement, an oblique 

 peristome, and similar phyllodes, but being slightly more advanced 

 in having a marginal periproct. Hypopyguriis appears to be an 

 offshoot of Pygorhynchus in having similar petals and the same 

 position of periproct, but it has buccal pores and is therefore more 

 advanced. 



The family Faujasidae appears to be descended from the Nucleo- 

 litidae, probably from a form similar to Phyllohrissus. The evolu- 

 tionary trend in this family seems to be toward a decrease in the 

 number of pores in the phyllodes, an increase in the development of 

 the bourrelets and petals, and the change from a tetrabasal to mono- 

 basal apical system. Petalobrissus is similar to Phyllohrissus in hav- 

 ing the same shape and a supramarginal periproct, but is more ad- 

 vanced in having single pores in the ambulacral plates beyond the 

 petals, buccal pores, stronger bourrelets, and wider outer poriferous 

 zones in the petals. Hardouinia originated from an ancestor like 

 Petalobrissus. The older species of Hardouinia are very similar to 

 some species of Petalobrissus, having similar phyllodes, a supra- 

 marginal, longitudinal periproct, and a similarly shaped test. They 

 differ only in that the bourrelets are more strongly developed in 

 these species of Hardouinia and the periproct is more central. Later 

 species of Hardouinia have much wider, shorter phyllodes with even 

 more developed bourrelets, and are easily distinguished from Petalo- 

 brissus. Pygidiolampas is an offshoot from Hardouinia, differing 



