202 ROBERT TRACY JACKSON ON ECHINI. 



mouth in the peristome. Primordial interambulacral plates in the basicoronal row, or usually 

 resorbed. Base of corona not resorbed, or usually resorbed. Oculars all exsert or becoming 

 insert in the sequence I, V, or V, I, IV, II, III. Periproct plated with few or many plates, 

 or granules, or largely leathery. Peristome with ten primordial ambulacra!, also non-ambula- 

 cral plates, or in one family many rows of ambulacral plates only. Lantern erect or rarely 

 inclined; teeth grooved, or keeled; foramen magnum deep; epiphyses narrow, or else wide 

 and uniting in suture over the foramen magnum. Pits in the top of pyramids. Pyramids with 

 ridges on lateral wings. Perignathic girdle consisting of low or high apophyses, and auricles. 

 Peristomal gills, rarely with Stewart's organs in addition. Primary spines without a cortical 

 layer. Primary tubercles perforate or imperforate. Sphaeridia present. 



Order CENTRECHINOIDA. 



D. Teeth grooved. Epiphyses narrow, not meeting in suture over the foramen magnum. 

 Ambulacral plates simple or compound. Coronal plates imbricate or not. Primordial 

 interambulacral plates resorbed or retained in the basicoronal row. Base of corona 

 resorbed or not. Oculars all exsert, or becoming insert in the sequence I, V, IV, II, III. 

 Periproct with many plates or granules, or largely leathery. Lantern erect or inclined. 



Primary tubercles usually perforate Suborder AULODONTA. 



Ambulacral plates compound ventrally, simple above the mid-zone, or in some genera 

 compound throughout. Coronal plates thick, not imbricating. Base of corona 

 resorbed. Oculars all exsert, or one, or two may be insert. Periproct unknown. 



Peristome unknown. Lantern erect Family HEMICIDARIDAE. 



Ambulacral plates simple. Coronal plates thin, not imbricating. Base of corona 

 resorbed. Oculars large, insert. Periproct leathery, but partially plated. Peristome 

 with ten very large primordial ambulacral plates. Lantern erect. 



Family ASPIDODIADEMATIDAE. 



Ambulacral plates compound. Coronal plates not imbricate (Mesozoic), or more or 

 less imbricate (Recent). Base of corona resorbed. Oculars exsert, or one to all 

 insert. Periproct, more or less plated, to nearly leathery. Peristome with ten prim- 

 ordial ambulacral, also non-ambulacral plates. Lantern erect, or (Astropyga) 

 inclined. Stewart's organs slight, or absent Family CENTBECHINIDAE. 



Ambulacral plates compound. Coronal plates very thin, imbricate. Primordial 

 interambulacral plates in basicoronal row. Base of corona not resorbed. Oculars 

 insert, often separated from the genitals by interspaces. Genitals more or less split 

 by secondary sutures. Periproct leathery but partially plated. Peristome with 

 many rows of ambulacral plates only. Lantern inclined. Radial peristomal and 



somatic muscles. Stewart's organs Family ECHINOTHURIIDAE. 



DD. Teeth keeled. 



E. Epiphyses narrow, not meeting in suture over the foramen magnum. Ambulacral 

 plates compound or largely simple. Coronal plates not imbricate. Primordial inter- 

 ambulacral plates resorbed or retained in the basicoronal row. Base of corona 

 resorbed or not. Oculars all exsert or becoming insert in the sequence I, V or V, 

 I, IV, II, III. Periproct with prominent suranal, or with many small plates, or 



