CLASS I 



ECHINOIDEA 



271 



Order 2 CIDAROIDA Duncan. 



Test regulär, endocycUc, two columns of plates in each ambulacral area, ambula- 

 cral plates low, simple; ftvo (in one genus, Tetracidaris, partly föur) columns of 



A, Bothriocidaris archaica Jackson. Ordovician ; Island of Dago, Russia. Height, 12 mm. Two rows of 

 peristomal plates. Two columns of hexagonal plates in each ambulacrum. One column of plates in each inter- 

 ambulacrum. x 3/j. B, the same, apical disk, oculars meeting in a ring, genitals small, dorsal to the oculars, peri- 

 proctal plates. x ^/i (after Jackson). 



plates in each interamhulacral area. Coronal plates rarely imbricate (Miocidaris). 

 Primordial interamhulacral plates resorhed. Peristome with many rows of ambulacral 

 and interradial non-ambulacral plates, or rarely ambulacral plates only. Lantern 

 erect, teeth grooved, foramen magnum very shallow ; epiphyses narrow. No pits in 

 the top of pyramids. Perignathic girdle consisting of apophyses only. Stewarts 

 Organs ptresent, but no peristomal gills. Primary spines with a cortical layer. 

 Primary tubercles perforate. Sphaeridia absent. 



