204 ROBERT TRACY JACKSON ON ECHINI. 



plates at mid-zone composed of from four to ten elements each, rarely (some 

 Echinostreplius) of three elements each. Oculars all ex.sert or becoming 

 insert in the sequence I, V, n', II. . . . Family Strongylocentrotidae. 

 Ambitus elliptical in a sidewise axis. No pits or sculpturing in coronal plates 

 dorsally. Ambulacral plates at mid-zone composed of four or more elements 

 each, rarely (Parasalenia) of three elements each. Oculars all exsert, or be- 

 coming insert in the sequence V, I, IV. . . . Family Echinometridae. 

 BB. Periproct outside of oculo-genital ring, in interambulacrum 5. Regular in form, or more fre- 

 quently more or less markedly bilaterally symmetrical through the axis III, 5. Slight or no 

 resorption of base of corona by the advance of the peristome. Lantern present or absent. Peri- 

 stomal gills, or ambulacral gills only. Sphaeridia present. . . Order EXOCYCLOIDA. 

 Ambulacral plates compound, or largel\' simple, areas not petaloid dorsally. Primordial 

 ambulacral plates imknown. Primordial interambulacral plates in basicoronal row, or in 

 part resorbed. Base of corona slightl.y resorberl. Oculars and genitals all present and dis- 

 tinct, or fused, or genital 5 absent; when present, it is imperforate. Periproct unknown. 

 Peristome central, structure unknown. Lantern inclined so far as known. Teeth keeled, 

 epiphyses narrow. Foramen magnum moderately deep. Pyramids with ridges on lateral 

 wings. Perignathic girdle consisting of low apophyses and auricles. Peristomal gills 



present Suborder HOLECTYPINA. 



Ambulacral plates simple, areas petaloid dorsally. \'entrally ambulacral pores are minute 

 and specialized. Primordial ambulacral plates in basicoronal row. Primordial inter- 

 ambulacral plates in basicoronal row, or exceptionally (Arachnoides, p. 72) pushed dorsally 

 and in part resorbed by intracoronal resorption. Base of corona not resorbed. Ocular anrl 

 genital plates fused in a mass, usualh- no genital pore in area 5. Genital pores witliin the 

 fused mass or outside in interambulacra 1, 2, 3, 4. Periproct plated. Peristome central, 

 leathery. Lantern procinnbent, highly modified, teeth keeled, foramen magnum very 

 shallow, small epiphyses and liraces, but no compasses. Pyramids usually without ridges 

 on lateral wings. Perignathic girdle consisting of auricles only, on ambulacral, or on inter- 

 ambulacral plates. No peristomal, but ambulacral gills only. 



Suborder CLYPEASTRFX.l. 



Ambulacral plates simple, areas commonly petaloid dorsally; in some types pores are absent 

 in part of the plates. Ambulacrum III often differs markedly in character from other 

 areas. Ambulacral plates often highly specialized in form and size. Primordial ambulacral 

 plates in the basicoronal row, or (Pourtalesia) in part pushed dorsally. The basicoronal 

 plates la, Iln, lllb, TVa, Yb are larger and with two pairs of pores or two separate single 

 pores, whereas the 16, lift, II I«, IV6, Va are smaller with one pore-pair or one 

 single pore. Primordial interambulacral plates in basicoronal row or (Lovenia, Pourtalesia) 

 pushed dorsally. Base of corona not resorbed. Oculars and genitals separate, or genitals 

 partially fused. Oculars apparently absent in some Pourtalesias. Genital 5 absent, and 

 some additional genitals rarely absent. Periproct plated. Peristome eccentric, plated with 

 non-ambulacral plates onl\'. Lantern and perignathic girdle absent (present in the young 

 of Echinoneus, pp. 189, 217). No peristomal, but ambulacral gills only. 



Suborder SPAT AN GIN A. 



