478 GEOLOGICAL DISTRIBUTION OF ECHINODERMATA. 



1117. The Pentacrinus first began to abound at the com- 

 mencement of the Lias ; and its remains present themselves in 

 great numbers in that formation, and in the Oolite which suc- 

 ceeds it. From that period they diminish, and we gradually 

 lose sight of this interesting group, the few existing species 

 serving, as it were, but to indicate the structure of those, which 

 formerly occupied so important a rank among the inhabitants of 

 the ocean. As the Crinoidea disappeared, the true Stellerida 

 and Echinida took their place ; and remains of these are met 

 with in almost all marine strata, down to the present time, 

 being particularly abundant in the Chalk and Oolite. 



