234 



ECHINODERMATA ECHINOIDEA 



SUB-BRANCH III 



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. 



A, B, Holectypus oriftcatus, Schlot. Upper Jura ; Streitberg, Franconia. C, D, H. depressus, Leske sp. 

 Middle Jura ; France. Apical system and ventral surface (after (Jotteau). 



Tubercles small, irregularly arranged. Periproct supra - marginal, small, 



broadly ovoid. Upper Jura. 



Pygaster, Ag. (Fig. 376). Test large, depressed dorsally, concave actinally. 

 Amb straight, similar, flush or slightly raised, 

 widest at the ambitus. Poriferous zones 

 straight, simple, narrow ; tubercles of interpor- 

 if erous areas in two or four vertical rows ; those 

 of the lAmb in horizontal rows. Peristome 

 large, decagonal, with jaws and feeble perignathic 

 girdle. Periproct immediately beyond the apical 

 system. Jura and Cretaceous. 



SECTION II. Perignathic girdle with well- 

 interambulacral ridges; ambulacral pro- 

 cesses rudimentary or absent. 



Discoidea, Klein (Fig. 377). Test hemi- 

 spherical above the margin, flat actinally. Amb 

 narrow, with some compound plates near the 

 ambitus and actinally ; pairs of pores very 

 numerous, small. lAmb with distinct median 

 sutures, and small, perforate, and crenulate 

 tubercles. Plates within the actinal surface 

 Young individual, with radiating ribs, ten in all, extending as far 

 as the peristome; appearing on casts as deep 

 Periproct small, infra-marginal. Cretaceous. 



FIG. 376. 



Pygaster umbrella. Ag. Oxfordian ; 

 Chatillon-sur-Seine. ' 

 1/1 (after Cotteau). 



depressions. 



Discoidea cylindrica, Ag. Upper Cretaceous; Liineburg. A, Side-view. B, Test broken open to show the 

 inner partitions, 1/1- 



Conoclypeus, Ag. (Fig. 378). Test large, thick ; conical or vaulted dorsally, 



