SYSTEMATIC CLASSIFICATION OF ECHINI. 205 



AA. Two or more columns of simple plates in each ambulacral area, and three or more columns of plates 

 in each interambulacral area (for A, see p. 201). 



B. Genitals largely covering the dorsal surface. Two columns of low plates in each ambulacral 

 area and three columns of plates in each interambulacral area. Plates not imbricate. Prim- 

 ordial interaml)idacral plates in basicoronal row. Base of corona not resorbed. Oculars small, 

 strongly exsert by the contact of large genitals. Periproct central, structure unknown. Peri- 

 stome central, structure unknown. Lantern and perignathic girdle unknown. 



Order PLESIOCIDAROIDA. 

 BB. Genitals small. Two to twenty columns of plates in each ambulacral area and three to four- 

 teen columns of plates in each interambulacral area. Plates not imbricating, or imbricating; 

 when the latter, ambulacral plates imbricate ventrally and interambulacral plates dorsally and 

 from the center laterally and over the ambulacra on the adradial sutures. Primordial ambulacral 

 plates on the peristome. Base of corona resorbed or not. Peristome central with many rows of 

 ambulacral plates only, or the same with non-ambulacral plates. Lantern inclined, composed of 40 

 pieces, teeth grooved. Foramen magnum moderately deep, epiphyses narrow. No pits in top of 

 pyramids. No perignathic girdle. No peristomal gills. Primary spines and perforate tubercles 

 with secondary spines and imjierforate tubercles, or secondaries only. 



(". Irregular, periproct apparently in an interambulacrum. Two to four columns of plates 

 in an ambulacral area, anil eight to nine columns of plates in an interambulacral area. Plates 

 thin, imbricating. Oculars and genitals doulttful. . . . Order ECHINOCYSTOIDA. 

 Two columns of plates in an ambulacral area and eight to nine colunms of plates in an 

 interambulacral area. Primordial interambulacral plates in basicoronal row. Base of 

 corona not resorbed. Peristome with ambulacral plates only. Lantern typically 

 echinoid. Numerous fine spines. (One genus and species.) Family Palaeodiscidae. 

 Four columns of plates in an ambulacral area, and eight columns of plates in an interam- 

 bulacral area. Small primary spines and tubercles. (One genus and species.) 



Family Echinocystidae. 

 CC. Regular, periproct within oculo-genital ring. Two to twenty columns of plates in each 

 ambulacral area, and three to fourteen columns of plates in each interambulacral area. 

 Plates imbricate or not. Primordial interambulacral plates in basicoronal row, or resorbed. 

 Base of corona not resorbed or resorbed. Oculars usually all insert. Genitals typically 

 with more than one pore each; rarely (Lepidechinus) with one pore each. Madreporite 

 usually not recognizable. Periproct covered with many thick plates. Peristome with 

 many rows of ambulacral plates only, or in addition with interradial non-ambulacral plates. 

 Spines primary and secondary, or the latter only. Primary tubercles perforate, secondary 



tubercles imperforate Order PERISCHOECHINOIDA. 



D. Two columns of plates in each ambulacral area, four to fourteen columns of plates 

 in each interambulacral area. Plates thin, imbricating. 



E. Four to eight columns of plates in an interambulacral area. Primordial and 

 additional interambulacral plates resorbed in the advance of peristome. Ocu- 

 lars, genitals, and periproct imperfectly known. Peristome with many rows of 



