THE ECHINODERMATA 273 



6. CENTRECHINOIDA [DIADEMOIDA]. Regular echi- 

 noids in which there is a tendency to form compound 

 plates in the ambs, which are not much narrower than 

 the interambs Pe!taste$(]ur.-Cret.),Salenia(CTcet.-'Rec.), 

 and Acrosalenia (Jur.-Cret.) have a large apical disc with 

 an extra suranal plate (Fig. 78, /). Hemicidaris (Jur.) has 

 been described. In Pseudodiadema Qur.-Cret.) three 

 primaries are fused, but the pores remain uniserial except 

 close to the peristome. In Diplopodia (Jur.-Cret.j, with 

 the same type of compound plate, the pores are biserial 

 i.e., they form two vertical columns within each column 

 of plates ; while in Stomechinns (Jur.-Cret., Fig. 78, //) 

 they are tvisevial. 



In Cyphosoma (Jur.-Cret., Fig. 78, g) as many as six 

 plates, three primary and three demi-, may fuse into a 

 compound plate to carry the big tubercles, which are 

 imperforate (the last three genera having them per- 

 forate). 



Echinus (Cret.-Rec.), the common sea-urchin, has 

 plates compounded of two primaries and one demi- ; 

 pores triserial ; poriferous zone narrow ; tubercles and 

 radioles numerous and not very large. 



Pelanechimis (Jur.) and Echinothuria (Cret.) form a re- 

 markable group, of which there are living survivors, in 

 which the whole corona is flexible, the plates being 

 movably articulated by bevelled edges. 



- SUB-CLASS : Irregularia (Exocyclica). Sea-urchins 

 of this sub-class appear first near the top of the Lower 

 Jurassic (Aalenian stage), and the simplest of them, 

 Pygaster (Fig. 79, a), is very little removed from some 

 simple member of the Regular Centrechinoida, the peri- 

 proct having shifted to a position only just outside the 

 apical system. With this, however, are others with much 

 stronger irregularity (Hyboclypeus), the mouth having also 

 shifted forwards from its central position and having 



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