ECHINODERMATA. ECHINOIDEA. 79 



ORDER. EUECHINOIDEA. 



The test is always formed of twenty rows of plates, 

 two in each area. The plates very rarely overlap. 

 Regular and irregular forms occur. There is usually 

 one pore only in each genital arid ocular plate. Jaws are 

 present in some forms, absent in others. 



(a) Regular Euechinoids. 



The test is radially symmetrical and generally sphe- 

 roidal. The apical disc encloses the anus. The mouth 

 is central. The ambulacral areas are simple. Jaws 

 present. 



Cidaris. Test spheroidal, the summit and base 

 equally flattened. Apical disc very large, rarely preserved 

 fossil, ocular plates large. Ambulacral areas narrow, 

 undulating, pores unigeminal, between the rows of 

 pores are vertical rows of small tubercles and granules. 

 Interambulacral areas wide, plates few, each with a 

 primary tubercle which is generally perforated, and may 

 be crenulated or smooth ; areola large, surrounded by 

 secondary tubercles, beyond which are granules. Peri- 

 stome rounded. Spines large, of various forms, generally 

 ornamented with rows of granules. Lias to present day. 



Peltastes. Test circular in outline, somewhat de- 

 pressed. Apical disc very large, prominent, containing 

 one extra plate, the sur-anal, placed in front of the 

 periproct ; the madreporic plate has an oblique fissure. 

 Ambulacral areas narrow, straight or slightly flexuous, 

 with small tubercles; pores unigeminal except near the 

 mouth. Interambulacral areas wide, with large primary 



