ECHINODERMATA. ECHINOIDEA. 83 



found almost exclusively in the Palaeozoic, only one genus 

 occurring in the Neozoic, viz. Anaulocidaris from the 

 Trias. The Euechinoids are nearly confined to the 

 Neozoic ; Eocidaris from the Permian, being the only 

 genus found in the Palaeozoic. 



The earliest echinoid is the genus Bothriocidaris from 

 the Ordovician rocks of Esthonia in Russia. In the 

 Silurian we get Palceechinus and Echinocystites ; in the 

 Devonian, Lepidocentrus; in the Carboniferous, Palceechinus, 

 Melonites, and Archceocidaris. In the Jurassic the 

 echinoids are much more numerous, relatively to the 

 other groups of animals, than in the earlier formations, 

 they are rare in the Lias and the other clayey divisions, 

 but very abundant in the calcareous beds, especially in 

 the Inferior Oolite and Coral Rag. The genera which are 

 best represented in the Jurassic are, of the regular group, 

 Gidaris, Hemicidaris, Acrosalenia, Pseudodiadema, and 

 Stomechinus ; of the irregular group, Collyrites, Echino- 

 brissus, Holectypus, and Pygaster. In the Cretaceous the 

 echinoids are even more abundant than in the Jurassic ; 

 many of the genera found in the Lower Cretaceous occur 

 also in the Upper Jurassic. The most important genera 

 are, (1) regular, Cidaris, Pseudodiadema, Cyphosoma, 

 Peltastes, Salenia, (2) irregular, Discoidea, Echinoconus 

 (= Galerites), Hemiaster, Micraster, Cardiaster, Holaster, 

 Echinocorys (= Ananchytes). Between the Cretaceous 

 and the Eocene there is an enormous break in the 

 succession of the echinoids, not a single species being 

 common to the two systems, and the genera also are 

 mostly different. This change is due in part to the great 

 difference in the conditions under which the deposits were 

 formed, the Chalk being a comparatively deep water 

 formation, and the Eocene beds, shallow water; but the 



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