134 ECHINODERMA. ECHINOIDEA 



Jurassic are, of the regular group, Gidaris, Hemicidaris, 

 Acrosalenia, Pseudodiadema, Diplopodia, Hemipedina, and 

 Stomechinus ; of the irregular group, Gollyrites, Clypeus, 

 Pygurus, Hyboclypeus, Nucleolites (= Echinobrissus), Ho- 

 lectypus, and Pygaster. 



In the Cretaceous the echinoids are even more abundant 

 than in the Jurassic, and attain their greatest development 

 in the upper division of the system ; many of the genera 

 found in the Lower Cretaceous occur also in the Upper 

 Jurassic, but the irregular forms are more numerous than 

 hitherto. The most important genera are, (1) regular, 

 Cidaris, Pseudodiadema, Phymosoma (= Gyphosoma), Pel- 

 tastes, Salenia ; (2) irregular, Discoidea, Galerites (= Echi- 

 noconus), Hemiaster, Micraster, Gardiaster, Holaster, 

 Echinocorys (= Ananchytes). 



Between the Cretaceous and the Eocene there is, in 

 Britain, a great break in the succession of the echinoids ; not 

 a single species is common to the two systems, and most of 

 the genera also are 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 Eocene forms differ much more from those of the 

 Upper Chalk than from those of the Chalk Marl, the 

 latter deposit having been formed in water of less depth. 

 Throughout the English Tertiaries the echinoids are much 

 rarer than in the Cretaceous; thus in the Cretaceous 

 there are thirty genera, in the Eocene seven, in the Pliocene 

 eleven. In the Eocene this can be accounted for largely 

 by the fact that the sea-bottom was for the most part 

 muddy ; in the Pliocene, by the lower temperature of the 

 ocean. The London Clay echinoids belong to tropical or 

 sub-tropical genera. The commonest forms in the Eocene 



