CLASS I ECHINOIDEA 283 



Family 4. Echinometridae Gray. 



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

 'hrsalhj. Ambulacral plates at mid-zone composed of four or more elements each, 

 rarely (Parasalenia) of three elements each. Oculars all exsert, or becoming insert 

 in the sequence V, I, IV. Recent. 



Echinometra Gray ; Heterocentrotns and Colobocentrotus Brandt ; with highly 

 specialised spines. Recent. 



Order 4. EXOCYCLOIDA Jackson. 



Test irregulär, exocyclic, periproct outside of oculogenital ring in interambulacrum 

 -7. Two columns of plates in each ambulacral area and two columns of plates in 

 each interambulacral area. Regulär in form, or more freguently 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. 

 Peristomal gills, or ambulacral gills only. Sphaeridia present. Jura to Recent. 



This Order includes all exocyclic Echini excepting the Paleozoic 

 Echinocystoida. The order shows structural characters associating it with 

 the Arbaciidae. 



SuborderA. HOLECTYPINA Gregory. 



Ambulacral plates Compound, or largely simple, areas not petaloid dorsally. 

 Primordial ambulacral plates unknown. Primordial interambulacral plates in basi- 

 coronal row, or in part resorbed. Base of Corona slightly resorbed. Oculars and 

 genitals all present and distinct, 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 con- 

 sisting of apophyses and auricles, or auricles only. Peristomal gills present. Jura 

 to Tertiary. 



Family 1. Discoidiidae Gregory. 



Perignathic girdle consisting of apophyses and auricles. 



Discoidea Gray (Fig. 397). Test hemispherical above the margin, flat 



Discoiiha cylindrica Agassiz. Upper Cretaceous ; Lüneburg. A, Side-view. B, Test broken open to show 



the inner partitions, i/i. 



actinally. Amb narrow, with some Compound plates near the ambitus and 

 a>ctinally ; pore-pairs very numerous, small. lAmb with distinct median 



