30 PALAEONTOLOGY 



with smooth, solid spines (== Hemidiadema), appear in the 

 lias, and continue to the chalk, where the modern type, with 

 annulated, hollow spines, appears. Echinopsis also occurs in 

 the lias ; and Acrosalcnia, a genus characteristic of the oolites, 

 and distinguished from Scdenia by its perforated tubercles. 

 Acrocidaris and Heliocidaris, with Glyjrticus, and several other 

 sub-genera of Echinus, are also peculiar to the oolites. Scdenia 

 (fig. 8, 5) with its ornamental disk, is characteristically creta- 

 ceous. Arhacia and Tcmnoplcurus appear first in the eocene. 

 The Cassidididw commence in the oolites, with Pygastcr (fig. 

 7, 1) and Hokctyjnis, and abound in the cretaceous system. 

 Gcdcrites (fig. 7, 3), Discoidca, Pyrina, and Cassidulus are pecu- 

 liar to the chalk. The Clypcastridw are represented in the 

 oolites by numerous species of Echinolampas and Nucleolitcs 

 (or Clypeus) ; the latter genus attains a large size. The sub- 

 genus Catopygus (fig. 8, s) is peculiar to the cretaceous series. 

 Conoclypeus occurs in the chalk and tertiaries. Ch/pcaster 

 flourished most in the miocene age ; many large species are 

 found in the south of Europe, Madeira, and the \Vest Indies. 

 Numerous genera, remarkable for their flattened form, and 

 popularly known as " cake-urchins," are peculiar to the ter- 

 tiaries and existing seas. Lcnita and Scutcllina are eocene ; 

 Scutella (fig. 7, 4) is miocene. Mellita and Echinavachnius 

 are both fossil and recent. The heart-shaped urchins {Spatan- 

 gidce), are only remotely represented in the oolites by Disaster 

 (fig. 8, 6) ; they are numerous in the chalk, to which Micraster, 

 Epictster, Hcmipineustcs (fig. 8, 7), ArcMada, Holaster, and 

 Ananchytcs (fig. 7, 2), are peculiar. Toxastcr is characteristic 

 of the lower neocomian. llaniaster is cretaceous and tertiary. 

 Spafauytix, Evpatagus, Brissus, Amphidotus, and Schizaster are 

 tertiary and recent forms. 



The shell of the Echinod&rm-ata lias the same intimate 

 structure in all the orders and families, and in every part of 

 the skeleton, whether "test," or "spine," or "tooth." The 



