THE ECHINODERMATA 



277 



from the other ambs. Symmetry strongly bilateral. 

 Tendency to separate the three anterior ambs (triviuin) 

 from the two posterior (bivium). 



There are two sub-orders (i) Cassiduloidea, in which 

 the mouth, though slightly forward, still opens down- 

 wards; and (2) Spatangoidea, in which the mouth opens 

 forwards, with more or less of an underlip, and between 

 it and the anus the posterior interamb plates form a 

 strong convex plastron. 



Chief genera of (i) are Nucleolites [Echinobnssns] (Jur.- 



FlG. 8l. ECHINOCORYS SCDTATOS, LESKE, EMSCHERIAN 

 (UPPER CHALK). 



a, Side view; b, apical disc; c, oral view. (Allx J.) (After Wright.) 



Cainozoic), with oval outline, broader and rather trun- 

 cated behind, with deep anal groove on upper surface, 

 ambs sub-petaloid, the outer pore of each pair elongated ; 

 Clypeus (Fig. 79, c, Jur. ), flat, circular, anal groove as in 

 the last, ambs petaloid, elongation of outer pores very 

 great, mouth surrounded by a floscelle five ambulacral 

 grooves (phyllodes), with protuberances (bourrelets) on the 

 interambs; Echinolampas (Eoc.-Rec.), ovoid, somewhat 

 conical, ambs petaloid, anus sub-marginal, floscelle 

 present ; Collyrites (Fig. 80, Jur.-Cret), oval, with rounded 

 base, with strong separation of bivium and trivium. 



