226 PALEONTOLOGY. 



The SPATANGOID^:, of which Micraster and AnancTiytes are 

 types, appeared at the close of the oolitic, and attained a 

 great development in the seas of the cretaceous period, the 

 rocks of which contain a great variety of species very nume- 

 rous in individuals.* 



The CIDAKID^; are the most highly organised, and, at the 

 same time, the most ancient fainily.t They commenced 



in the permian group 

 and the muschelkalk, 

 lived in the seas of 

 the lias, but had 

 their greatest deve- 

 lopment in the oolitic 

 period. Of all the 

 echinidae, this family 

 has relatively dimin- 

 ished most in the 

 tertiary and modern 

 epochs. 



FIG. 158. Cidaris margaritifera. The ASTEEIDA, Or 



sea-stars, have a disc- 



oidal body composed of a central axis with branching move- 

 able rays, more or less elongated, generally five in number, 

 and either entire or branched. The skeleton is composed of 

 many hundreds of calcareous pieces enveloped in an organised 

 integument. The nervous system consists of a circular 

 filament which surrounds the entrance to the stomach, and 

 has a pair of ganglia, and a lash of nerves developed for each 

 ray. The mouth is edentulous, inferior and central, and the 

 body is free and ambulatory. J 



It comprises three families, two of which, the ASTERIADJE 

 and OpHiURiixas, are represented by a few species in the 

 oolitic and cretaceous rocks. 



The CEIKOIDA, or sea-lilies, are thus defined by Millar : 

 " An animal with a round oval or angular column, composed 

 of numerous articulating joints, supporting at its summit a 

 series of plates or joints, forming a cup-like body containing 

 the viscera, from whose upper rim proceed five articulated 

 arms, dividing into articulating fingers more or less nume- 



* Pict. Atlas, pi. lv. t Pict. Atlas, pi. liii. 



J Pict. Atlas, pi. liii. fig. 3. Natural History of the Crinoidea. 



