274 



HISTORICAL PALEONTOLOGY. 



present day adhering to the exterior of shells. The occurrence 

 of this genus is of interest, because it is the first known instance 

 in the entire geological series of the occurrence of an unques- 



Flg. IQO.Synfielia Sharpeana. Chalk, England. 



tionable Hydroid of a modern type, though many of the exist- 

 ing forms of these animals possess structures which are per- 

 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- 

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

 the same great group of corals as the majority of existing 

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

 macis), hardly, if at all, generically separable from very ancient 

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

 remains of the little Plolocystis 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 Pentacrinus 

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

 our existing Comatulce; whilst a link between the stalked and 

 free groups is constituted by the curious " Tortoise Encrinite 

 (Marsupites). 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, especially 

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



