266 HISTORICAL PALAEONTOLOGY. 



fectly fitted for preservation in the fossil condition. The corals 

 of the Cretaceous series are not very numerous, and for the 

 most part are referable to types such as Trochocyathus, Stephano- 

 phyllia, Parasmilia, Synhelia (fig. 190), &c., which belong to 

 the same great group of corals as the majority of existing 



190. Synhelia. Sharpeana. Chalk, England. 



forms. We have also a few " Tabulate Corals " (Polytre- 

 maa's), hardly, if at all, generically separable from very ancient 

 forms (HeUolites); and the Lower Greensand has yielded the 

 remains of the little Holocystis elegans, long believed to be the 

 last of the great Palaeozoic group of the Rugosa. 



As regards the Echinoderms, the group of the Crinoids now 

 exhibits a marked decrease in the number and variety of its 

 types. The " stalked " forms are represented by Pentacriuus 

 and Bourgueticrinus, and the free forms by Feather-stars like 

 our existing Comatnla ; whilst a link between the stalked and 

 free groups is constituted by the curious " Tortoise Encrinite 

 (Mars unites). ' By far the most abundant Cretaceous Echino- 

 derms, however, are Sea-urchins (Echinoids) ; though several 

 Star-fishes are known as well. The remains of Sea-urchins are 

 so abundant in various parts of the Cretaceous series, especi- 

 ally in the White Chalk, and are often so beautifully preserved, 

 that they constitute one of the most marked features of the 

 fauna of the period. From the many genera of Sea-urchins 

 which occur in strata of this age, it is difficult to select char- 

 acteristic types; but the genera Galcritcs (fig. 191), Discoidea 

 (fig. 192), Micraster, Ananchytes, Diadema, Salenia, and Ci- 



