5 5 8 ECHINODERMATA ECHINOIDEA chai'. 



side by side, forming the interradial area of the corona, and one 

 large genital plate ; the ambulacra, of two rows of pore-plates. 

 This family consists of dwarfed forms which probably inhabited 

 the- land-locked seas and salt lagoons of the Triassic epoch. 



When we recollect that some of the oldest Asteroidea known 

 to us had very narrow arms and interradial areas edged by large 

 square marginals, it does not require a very great effort to 

 imagine how these marginals could be converted into the vertical 

 rows of the interambulacra, and the pointed narrow arms 

 becoming recurved, could have formed the ambulacra. The 

 physiological advantage of this will be discussed in the chapter 

 on development. 



True Cidaridae occur in the Permian, and are abundant in 

 all the younger formations. One Cretaceous genus, Tetra- 

 cidaris, has four rows of interambulacral plates near the mouth, 

 diminishing to two at the apex. This circumstance renders it 

 probable that the Cidaridae are the direct descendants of the 

 Archaeocidaridae. The Saleniidae, Echinothuriidae, and Diadema- 

 tidae appear in the Jurassic, the Echinidae in the Cretaceous, 

 and the Arbaciidae only in the Tertiary epoch. 



Turning now to the Mesozoic forms with an excentric anus, 



there were a number of forms which have been grouped together 



as Holectypoidea which had auricles and teeth and gills, although 



these were only feebly developed, and in which the pore-plates 



remained separate. The periproct was a comparatively large 



area, and in Pygaster, as in the surviving form Pygastrid,es, it 



.^.saeag,.^ was in contact with the apical 



^^^f '*"*^^ system, although outside it. 



A.^ Many of the genera were of con- 



/ ' ^a siderable height in proportion to 



\ mH iliiiBP'^^^l a- ^^^i^ length. In Gonoclypeus and 



i<^*i5™ '- ^''^^ Discoidea the jaws and auricles 



^-.V"^ ' -were very weak. The Echino- 



-' v^ ^ conidae have only vestigial aur- 



■^ icles, and on this account are 



■* -'^Usiii.-^'^ often definitely grouped with the 



Fig. 253.-Hybodypj^ gibbervius. x 1. Spatangoidea, but they are closely 



allied . to the Holectypoidea. 

 They are Cretaceous forms of high conical shape (Galerites). 

 In Hyhodypus (Fig. 253) all trace of the teeth has disappeared, 



