CLASS I 



ECHINOIDEA 



285 



ambitus ; some of the plates compound. lAmh with rather large plates, and 

 many rows of tubercles. Peristome large, decagonal, with well-marked branchial 

 incisions, jaws, and feeble perignathic girdle. 

 Periproct large, pyriform, situated between the 

 peristome and posterior edge of the test. Apical 

 system small, central. Jura and Cretaceous. 



Pileus Desor. Test large, sub-hemispherical 

 dorsally, flat actinally. Tubercles small, ir- 

 regularly arranged. Periproct supra-marginal, 

 small, broadly ovoid. Upper Jura. 



Pygaster Ag. (Fig. 400). Test large, de- 

 pressed dorsally, concave actinally. Amb straight, 

 similar, flush or slightly raised, widest at the 

 ambitus. Poriferous areas straight, simple, 

 narrow ; tubercles of interporiferous areas in 

 two or four vertical rows ; those of the lAmh in 

 horizontal rows. Peristome large, decagonal, 

 with jaws and feeble perignathic girdle. Peri- 

 proct immediately beyond the apical system. 

 Jura and Cretaceous. 



Galeroptjgiis Cotteau ; Pachydypeus Desor. 

 Upper Jura ; Europe. 



Pig. 400. 



Pyiiu-^ter umbclla Agassiz. Dxtbrdian ; 

 Chatillon-sur-Seine. Young indiviiUial, 

 Vi (after Cotteau). 



Suhorder B. CLYPEASTRINA Gregory. 



Ambulacral plates simple, areas petaloid dorsally. Ventrally amlndacral pores 

 are minute and specialised. Primordial ambulacral plates in basicoroual row. 

 Primordial interamhulacral plates in basicoronal row, or exceptionally (Arachnoides) 

 pushed dorsally and wi part resorhed by intracoronal resorption. Base of corona not 

 resorbed. Ocular and genital plates fused in a mass, usually no genital pore in area 

 5. Genital pores within the fused m,ass or outside in interambulacra 1, '2, 3, 4,. 

 Periproct plated. Peristome central, leathery. Lantern procumbent, highly modified, 

 teeth keeled, foramen magnum very shallow, small epiphyses and braces, but no com- 

 passes. Pyramids usually without ridges on lateral wings. Perignathic girdle con- 

 sisting of auricles only, on ambulacral, or on interambulacral plates. No peristomal, 

 but ambulacral gills only. Cretaceous to Recent. 



Family 1. Clypeastridae Agassiz. 



Test small to very large, depressed, flat or high. Petaloid parts of the ambulacra 

 highlif developed, usually uneqiud ; the actinal furrotvs straight. Interambulacra 

 actinally discontinuous ; one peristomal plate in each area. Perignathic processes 

 tall, narrow, two on each ambulacrum, fitting in below the large jaws. Peristome 

 central, pentagonal ; periproct small, marginal or infra-marginal. Internal structure 

 with needles, pillars, and other processes extending from floor to roof, especially near 

 the edge of the test ; sometimes these are fused to form concentric partitions, and the 

 ambulacra may also be protected by an inner wall. Tertiary and Recent. 



Clypeaster Lam. (Figs. 369, i^ ; 374, 401, 402). Actinal surface flat, with the 

 peristome deeply sunken ; edge thin, undulating in contour, with or without 



