206 ROBERT TRACY JACKSON ON ECHINI. 



ambulacral and interradial non-ambulacral plates. Spines large primaries with 

 perforate tubercles in the center of each interambulacral plate, also secondary 



spines and imperforate tubercles Family Archaeocidaridae. 



Fragmentarily known from dissociated interambulacral plates and spines. 

 Plates similar to those of Archaeocidaris but primary tubercles without a basal 



terrace Eocidaris (one species). 



Ambulacral plates all alike, of equal height. Four columns of plates in an 

 interambulacral area. Primary spines large, tapering, or inflated, smooth, 

 or with lateral spinules. Primary tubercles with basal terrace and scrobicular 



ring Archaeocidaris (many species). 



Ambulacral plates low, w'ith also higher, wider and wedge-shaped plates. Six 

 to eight columns of plates in an interamljulacral area. Primary spines cylin- 

 drical. Primary tubercles with no basal terrace, but with scrobicular ring. 



Lepidocidaris (one species). 

 EE. Five to fourteen columns of plates in an interambulacral area. Primordial 

 interambulacral plates in basicoronal row. Base of corona not resorbed. Ocu- 

 lars insert. Genitals with many pores each. Peristome with many rows of 

 ambulacral plates only. Spines small eccentrically placed primaries with secon- 

 daries, or the latter only Family Lepidocentridae. 



Test high, probably spheroidal; ambulacral plates high, two or three equaling 

 the height of an adambulacral plate, pore-pairs uniserial. Seven to eight 

 columns of nearly rhombic plates in an interambulacral area. 



Koninckoddaris (two species). 



Test high, spheroidal, ambulacral areas narrow throughout; ambulacral plates 



low, about eight equaling the height of an adambulacral plate, pore-pairs 



uniserial. Five to eleven columns of plates in an interambulacral area. Small 



primary spines and tubercles with secondaries on interambulacral plates. 



Lepidocentrus (five species). 

 Test depressed to flattened; tlirough the ambitus circular, pentagonal, or cly- 

 peastriform. Ambulacral areas broad, petaloid ventrally, narrow dorsally, 

 pore-pairs uniserial. Eleven to fourteen columns of plates in an interambula- 

 cral area. Small primary- and secondary tubercles on interambulacral plates. 



Hyattechinua (three species). 



Test high, spheroidal, ambulacral areas narrow throughout, pore-pairs mod- 

 erately biserial. Nine to ten columns of plates in an interambulacral area. 

 Secondary spines and tubercles only. . . . Pholidechinus (one species). 

 DD. Two to twenty (usually more than two) columns of plates in each ambulacral 

 area, three to eleven columns of plates in each interambulacral area. Plates not 

 imbricate or imbricate. 



E. Test elliptical, obovate, spherical or subspheroidal. Two to twelve col- 

 umns of plates in each ambulacral area, three to eleven columns of plates in each 



