SYSTEMATIC CLASSIFICATION OF ECHINI. 201 



In Palaeozoic types the given characters, especially of the peristome, may be known in only 

 one or a few cases, and further study may require modification in details. A genealogical 

 table is given (p. 209) that presents in a tree form the relations as understood, and following 

 this, as briefly as possible, a consideration of each of the several divisions, the fullest detail 

 being given of the Palaeozoic forms. Post-Palaeozoic genera of the regular Echini, and the 

 families of the Exocycloida are not included. 



Key to the Classification of the Echinoidea. 



I. Two columns of high hexagonal phites in each amhulacial area, and one cohinin of phites in each inter- 

 ambulacral area Order BOTHRIOCIDAROIDA. 



With characters of the order. • Family Bothriocidakidae. 



Ambulacral pores superposed in the center of plates. Plates not imbricate. Primordial ambulacra! 

 plates around mouth in tlie peristome. Primordial interambulacral plates in liasicoronal row. 

 Base of corona not resorbed. Oculars \ery large, genitals very small. Periproct with many 

 small plates. Peristome with two rows of ambulacral plates only. Jaws. 



Bothnocidari.b' (three species). 



II. Two or more colunuis of plates in each anibulaeral area, and two or more columns of plates in each inter- 

 ambulacral area. 



A. Two columns of simple or coinpoun<! plates in each amljulacral area, an<l two columns of plates in 

 each interambulacral area (for AA, see p. 205). 



B. Periproct within oculo-genital ring (for BB, see p. 204). 



C. Ambulacral plates low, simple; two (in one genus, Tetracidaris, partly four) columns of 

 plates in each interambulacral area. Coronal plates rarely imbricate (Miocidaris). Prim- 

 ordial ambulacral plates around the mouth in the peristome. Primordial interambulacral 

 plates resorbed. Base of corona resorbed. Oculars all exsert, or becoming insert in the 

 sequence ^', I, W, II, III, or ^', I, IV, III, II. Periproct covered with many thick plates. 

 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. Pyramids with ridges on lateral wings. 

 Perignathic girdle consisting of apophyses only. Stewart's organs present. Primary spines 

 with a cortical layer. Primary tubercles perforate. Sphaeridia absent. 



Order CIDAROIDA. 



With characters of the order. (Only one assured genus of this family occurs in the Palaeo- 

 zoic, therefore other genera are omitted.) Family Cidaridae. 



Interambulacral plates beveling over the ambulacral on the adradial suture. (Two 

 species only of this order and genus are definitely known in the Palaeozoic.) 



Miocidaris. 



CC. Ambulacral plates compound, rarely simple; two columns of plates in each interambulacral 

 area. Coronal plates not imbricate or imbricate. Primordial aml)ulacral plates around the 



