ECHINODERMA. ECHINOIDEA 133 



bottom is rocky, sandy, or calcareous, and less common 

 where it is muddy ; consequently fossil forms are rare in 

 clayey strata. Those found in deep water have a much 

 wider range in space than those found in shallow water. 

 Many genera, especially those with a considerable range 

 in depth, have also a long range in time, some extending 

 back to the Cretaceous or even earlier periods. Among 

 the forms found in the Cretaceous which are still existing 

 in deep water may be mentioned Salenia, Cottaldia, 

 Hemiaste?', and Pygaster. 



The Palaeozoic echinoids belong to the regular group ; 

 they are remarkable for possessing more than twenty 

 columns of plates in the corona (except in Bothriocidaris), 

 and frequently the plates overlap, rendering the test 

 flexible. Echinoids are rare in Palaeozoic formations, 

 especially in those of pre-Carboniferous age. The earliest 

 form is Bothriocidaris, from the Ordovician rocks of Esthonia 

 in Russia. In the Silurian, Echinocystis, Palceodiscus and 

 Palceechinus are found; in the Devonian, Lepidocentrus ; 

 in the Carboniferous, Palceechinus, Melonechinus (= Melo- 

 nites), Perischodomus, and Archteocidaris. Primitive 

 Cidaridas are common in the Trias of St Cassian and 

 Bakony. 



In the Jurassic rocks the echinoids are much more 

 numerous, relatively to the other groups of animals, than 

 in the earlier formations ; they are comparatively rare in 

 the Lias and the other clayey divisions, but very abundant 

 in the calcareous beds, especially in the Inferior Oolite and 

 the Corallian. In the Lias the chief forms are Cidaris,Hemi- 

 pedina, Diademopsis, Pseudodiadema, and Acrosalenia; 

 irregular echinoids (viz. Holectyp us, Collyrites, and Galero- 

 pygus) make their first appearance in the Upper Lias. The 

 genera which are best represented in the Middle and Upper 



