NO. 3 CASSIDULOID ECHINOIDS — KIER II 



petals so common in the Jurassic, and a predominance of species with 

 open petals and narrow outer poriferous zones such as in Pygaidus, 

 Catopygiis, Clypeopygus, and Pygorhynchus. In the Upper Creta- 

 ceous there is a tendency toward shorter, closed petals with broader 

 outer poriferous zones, such as in Petalobrissus, Zujfardia, Lcfortia, 

 Fanraster, and Faujasia. In the Maestrichtian Cassidulidae, the petals 

 are slightly developed and inconspicuous. A new feature is developed 

 in the Tertiary with the introduction of unequal poriferous zones in 

 the same petal. Usually the right poriferous zone in petal III is 

 longer than the left, the posterior zones in petals II and IV longer 

 than the anterior and the anterior longer in petals V and I. This 

 inequality in length is very common in the Tertiary, occurring in 

 almost all the species of Rhyncholampas, Cassididiis, and Echino- 

 lampas. The function of this inequality is not known. 



In almost every genus all the ambulacra are petaloid. The only 

 exceptions are Archiacia, Gcntilia, Claviaster, and Pseudopygaulus 

 in which ambulacrum III is nonpetaloid, or the petal is greatly re- 

 duced. Lambert thought that there was no petal III in Hctcronucleiis, 

 Pseudonudeus, and Pseudopygiirus, but I have examined the type 

 specimens of these genera under high magnification and found petal 

 III. There was probably a petal III in the adult of Sphclatiis, but 

 the type specimen is immature and the petal not developed. 



APICAL SYSTEM 



The apical system undergoes considerable evolution from the 

 Jurassic to the Recent, changing from a tetrabasal system with or 

 without complementary and catenal plates to a monobasal system 

 without any extra plates. In the Jurassic species, the system is tetra- 

 basal, has four genital pores, and may have (text figs. 30-35) many 

 complementary and catenal plates. The number and arrangement of 

 these plates are very variable, even down to the species level. In 

 some specimens of Clypeus agassisi (Wright), there are catenal plates 

 joining the apical system to the periproct, but in other specimens there 

 are none. Likewise, in Niiclcolites scutatus Lamarck catenal and 

 complementary plates may or may not be present. Early Cretaceous 

 species still have a tetrabasal apical system and four genital pores, 

 but there are no catenal or complementary plates. In the Late Creta- 

 ceous beginning with the Turonian, there are species with only three 

 genital pores. In the Senonian an abrupt change occurs, with many 

 species having monobasal apical systems with only one genital plate, 

 presumably genital 2. This change is very rapid and all inclusive for 



