ECHIN01DEA 



35 



secondary and more modem Echinidce, the shell is composed 

 of five double rows of ambulacral plates, and five inter-ambu- 

 lacral ; but in the Palcechinus (fig. 8, i) of the carboniferous 

 limestone there are six rows of inter-ambulacral plates, and in 

 Perischodomus five. Only detached plates of Arclwocidaii* 



Fig. 8. 

 Ecliinidoz ; Spatanyidie. 



i. Palajchinus spha?ricus, Scouler ; Carboniferous, Ireland. 



2. Archseocidaris Urii, Flem. ; Carboniferous, Ireland . 



3. Cidaris glandifera, Goldf. (spine) ; Jura, Mount Carmel. 



4. Henricidaris intermedia, Flem. ; Corallian, Calne. 



5. Salenia petalifera, Desm. ; U. Greensand, Wilts. 



6. Disaster ringens, Ag. ; Inferior Oolite, Dorset. 



7. Hemipneustes Greenovii, Forbes; U. Greensand, Blaekdown. 



8. Catopygus carinatus, Goldf. ; U. Greensand, Wilts. 



(fig. 8, 2), have been seen, and these, by their six-sided form, 

 seem also to have been arranged in more than double series. 

 Normal Echinida?, of the existing genus Cidaris, abound in 

 the upper trios, Some of the secondary species of Cidaris 

 have the ambulacral pores widely separated (=Rhabdoci- 

 daris) ; in others the rows of pores are doubled {== Diploci- 

 daris). The genus Hemicidaris (fig. 8, 4), distinguished by 

 the large spine-bearing tubercles on the lower part of the 

 ambulacra, ranges from the trias to the chalk-marl. Diademce, 



