i 34 SCIENCE PROGRESS 



adequately under changed conditions of environment. Similarly 

 the term Palaeoechinoidea is no longer held to denote a natural 

 group, for the Cidaroidea, from which all the post-Triassic and 

 living Echinoids can be derived, are clearly descended from the 

 late-Palaeozoic Archceocidaridce ; even in the Cretaceous period 

 the reduction of the rows of interambulacral plates to two 

 in number was not absolutely constant, for Tetracidaris has 

 four rows of such plates near the mouth, decreasing to two 

 at the apex. 



In corals, again, the old designation of Rugosa or Tetra- 

 coralla, confined to Palaeozoic forms, has proved to be equally 

 artificial, some of its members being now classified with the 

 Madreporaria, e.g. the Cyathophyllidce (which are probably 

 ancestral to both the Astrceidce and the Fuugiidce), and the 

 Cyathaxonidce (which are considered to have given rise to the 

 Turbinoliidce). The massive Zaphrentoidea, on the other hand, 

 have recently been shown by Duerden 1 to be closely allied 

 to the naked Zoanthidce (many of which are epizoic on Hermit 

 Crabs) and furnish another interesting instance of the abandon- 

 ment of armour and the survival of more flexible forms. 



1 Ann. Mag. Nat. Hist. 7, ix. p. 381, 1902. 



