276 PALAEONTOLOGY 



radiating partitions to be developed internally to support 

 the roof. The other is for the margin to be notched in the 

 interambs, the plates in the sides of these notches serving 

 as supports of the roof : further growth may reunite the 

 plates at the margin, converting notches into perforations 

 (" lunules "). In all but the more primitive genera some 

 of the amb plates widen out and pinch the interambs 

 across until they meet the plates of the next amb ^dis- 

 continuous interambs), and there are furrows (actinal 

 furrows) radiating from the mouth along the ambs. 



Chief genera : Echinocyamus (Cret -Rec.) and Fibularia 

 (Cret.-Rec ) are very small primitive forms with petaloid 



FlG. 80. COLLYRITES RINGENS, AGASSIZ, VESULIAN. 



Aboral view. (Xf.) Ptriproct seen at lower end. The four black spots 

 near centre are the genital pores of the apical disc ; from here the 

 " trivium " radiates, while the " bivium " is far behind. (After 

 Wright.) 



ambs, but none of the other special faatures of the sub- 

 order. Scutella (Eoc.-Rec.) is very flat, circular, with 

 bifurcating actinal furrows ; Mellita and Rotula (Plio.~ 

 Rec.) have lunules; Clypeastev (Fig. 79, d, Olig.-Rec.) is 

 oval to pentagonal in plan, with straight actinal furrows 

 in some species advantage is taken of the strengthening 

 pillars to raise the central part of the roof, including the 

 respiratory petals, into a dome. Such species are the 

 largest and most massive of all sea-urchins. All but 

 the most primitive genera of this sub-order are confined 

 to warm seas. 



ORDER 2 : ATELOSTOMATA. Mouth shifted forward ; 

 lantern and jaws lost. Anterior amb somewhat different 



