262 



PALEONTOLOGY 



The five areas containing pores are called the ambu- 

 lacral areas, or, briefly, the ambs ; the alternate areas are 

 the mterambulacral areas or interambs. The ambs are 

 perradial, and the interambs interradial. Together they 

 form the corona, which is the greater part, but not the 

 whole, of the test. 



It is not always easy to see the sutures of the plates 



FIG. 75. HEMICIDARIS INTERMEDIA, FLEMING, ARGOVIAN 

 (CORAL RAG), CALNE (WILTS). 



a, Aboral view. (x.) All details of ambs and interambs omitted. 

 b, Apical disc. (Natural size. ) c, An interamb plate in profile. (X2.) 

 d, Two interamb plates with adjacent part of amb. ( X 2. ) e, Large 

 rndiole. (Xf.-) (After Wright.) A, Periproct ; Ac., acetabulum 

 (articular socket for mamelon) ; Amb., ambulacral area; B, boss; 

 G, genital plate; /.A., interambulacral area; M, mamelon; Mad., 

 madreporite ; Oc. , ocular plate. 



which build up the ten areas, especially on the external 

 surface. On the internal surface they are plainer. Each 

 area consists of two vertical columns of plates, the sutures 

 down the middle of an area being zigzag, while those 

 between adjacent areas are nearly straight. In the 

 interambs there is a plate to each large tubercle. In the 

 ambs there is, near the aboral pole, one pore-pair to each 



