56 INVERTEBRATE ANIMALS. 



Family CLYPEASTKID^. chjpeus, a shield; astrum, 

 a star. 



Group 151. — Genus ECHINARACHNIUS (Van Phels) 

 and allies. ?>ihog, apuKVYj, a spider's web. The globose 

 form of the common Sea-urchin differs very widely from 

 the flattened disc of the Cake-urchin, EchinaracJinius 

 placenta^ a northern form rarely found in British seas ; 

 but upon examination the true ambulacral pores of the 

 Cake-urchin may be seen, forming a rosette on its 

 upper surface ; and other very curious modifications of 

 the ordinary echinoid structure may be recognised as 

 associated with an extreme reduction in the size of the 

 body cavity of the animal. Estimated number of 

 species : recent, 32 ; British, 2 ; fossil, 63. 



IT Examples of Melitta, Dendraster, Rumphia, 

 Lohophora, Scutella. 



Group 152.— Genus CLYPE ASTER (Lamarck) and allies. 

 Shield-stars. Between the Cli/peastridce of the present 

 and the Spatangidcs of the succeeding group occurs the 

 family Cassidulida, in which the mouth is central, but 

 the dental apparatus is wanting. The species, of which 

 M. Dujardin recognises more than 260, begin to appear 

 in the lower Oolite, attain their maximum in the Chalk, 

 diminish progressively in the Eocene and Miocene, 

 whilst the living representatives are confined to two or 

 three species found in tropical seas. Estimated number 

 of species : recent, 18 ; British, ; fossil, 285. 



IT Examples of RoUda, Encope, PygorJiynchus, &c. 



Group 153.— Family SPATANGID^. aTruruyyog, a Sea- 

 urchin. The Spatangidse, or Heart-urchins, live in 



