GENERA AKD GROUPS OF THE EGHINOIDEA. 159 



branching further out ; the median furrow continues to the 

 margin and may also be abactinal. 



Interradia narrow, but the two apical plates do not embrace 

 the genital pores, which are on the flanks of the madreporite ; 

 actinally the posterior interradium is discontinuous in the adult, 

 Perignathic processes single, upon an interradial peristomial plate. 



Peristome central, its margin composed of ten ambulacral and 

 five interradial plates ; buccal tentacles large, simple. 



Periproct actinal, marginal or supramarginal. Marginal pedi- 

 cellarise bifid. A single covered spheridium in each ambulacrum. 



Internal structures close, broad, concentric, discontinuous 

 partitions and pillars, which radiate from points and occupy 

 much space near the inner edge ; more or less continuous parti- 

 tions limiting the ambulacra for some distance inwards. 



Recent. Both sides of N. America, Japan, Kamtschatka, Aus- 

 tralia, New Zealand, Indian Ocean, Eed Sea. 



Genus Echinodiscus, Breynius, 1732, Sc7iediasma,-p- 63, tab. vii. 

 (pars). Agassiz, 1841, Monogr. d^Ecli.tiv. et foss. Iiyw 2, 

 pp. 62, 72 ; Gra^, 1855, Cat. Sec. EcTi. Brit. Mus. pt. i. 

 p. 19. A. Agassiz, 1873, Bevision, p. 531. 



Syn. Ampliiope, Agass., 1841 ; Lohophora, Agass., 1841. 



Test moderate and large, thin, flat, slightly raised dorsally, 

 subcireular, widest posteriorly and truncated there. Two lunules 

 or slits posteriorly, one in each of the median lines of the postero- 

 lateral ambulacra ; lunules long, narrow, or broad and circular. 



Apical system central or t^lightly anterior. Madreporite 

 central, large, stellate ; four genital pores beyond ; radial pla^^es 

 small. Ambulacral petals small, broad, unequal, closed ; the 

 poriferous zones very broad ; actinally the grooves are single near 

 the peristome, perforate, wavy, and they soon bifurcate and again 

 near the margin ; a buccal process in each groove near the 

 peristome. 



Interradia narrow, the two apical plates of each having a 

 genital pore in their median suture. 



Peristome small, central, lobed or subpentagonal. Jaws flat, 

 stellate, teeth horizontal, placed above the perignathic processes, 

 which are single in each interradium and low and oblique. Peri- 

 proct small, actinal, nearer the posterior edge of the test than 

 the peristome. 



Internal structure with the central space free, and beyond to 

 the inner edge there are stellate networks of cellular supports 



