84 ECHINODERMATA. ECHINOIDEA. 



Eocene forms differ much more from those of the Upper 

 Chalk than they do from those of the Chalk Marl, the 

 latter being a shallower water deposit. Throughout the 

 English Tertiaries the echinoids are much rarer than in 

 the Cretaceous ; thus in the Cretaceous there are thirty 

 genera, in the Eocene seven, in the Pliocene eleven. In 

 the Eocene this can be accounted for largely by the fact 

 that the sea-bottom was for the most part muddy ; in the 

 Pliocene by the lower temperature of the ocean. The 

 London Clay echinoids belong to tropical or sub-tropical 

 genera. The commonest Eocene forms are Hemiaster and 

 Schizaster. The Pliocene forms present a considerable 

 affinity to those now living in the West Indian seas, 

 indicating the existence of shallow water between Europe 

 and America in which migration could take place. The 

 most important genera in the Pliocene are Echinus, 

 Echinocyamus, Spatangus, and Temnechinus. 



CLASS. HOLOTHUROIDEA. 



This includes the sea-cucumbers. They possess an 

 elongated cylindrical body provided with a circle of 

 tentacles around the mouth connected with the water- 

 vascular ring ; from the radial vessels tube-feet are given 

 off, except in the genus Synapta. The integument is 

 leathery, and the calcareous skeleton is very poorly develop- 

 ed, consisting of minute isolated pieces of various shapes, 

 such as spicules, anchors, and wheels. At the present day 

 the Holothurians are widely distributed, but owing to the 

 nature of their hard parts, they are rarely found fossil. 

 The earliest forms occur in the Carboniferous rocks of 

 Scotland, and a few have also been recorded from the 

 Jurassic and later formations. 



