ECHINODERMA. ECHINOIDEA 129 



symmetrical. Ambulacra simple or petaloid. Jaws may 

 be present or absent. 



SUB-ORDER 1. GNATHOSTOMATA 



Mouth central. Jaws and perignathic girdle present. 



Galerites ( = Echinoco?ms, Gonulus) (fig. 47 E). Test conical, 

 or almost hemispherical, inferior surface flat, outline pentagonal or 

 oval. Apical disc small. Ambulacra narrow, straight, simple ; 

 pores unigeminal, but trigeminal near the mouth. Interambulacra 

 with broad plates, tubercles very small, perforated and crenulated. 

 Peristome small, central, decagonal. Periproct marginal or sub- 

 marginal. Upper Greensand to Upper Chalk. Ex. G. conicus, 

 Upper Chalk. 



Hole cty pus. Test hemispherical, depressed, base excavated. 

 Apical disc small. Ambulacra narrow, straight, simple ; pores uni- 

 geminal, tubercles small. Interambulacra formed of rather large 

 plates, with small tubercles. Peristome central, decagonal. Peri- 

 proct large, placed between the peristome and the posterior margin 

 of the test. Upper Lias to Corallian ; also foreign Cretaceous. 

 Ex. H. kemisphericus, Inferior Oolite. 



Discoidea. Form similar to Holectypus. On the base of the 

 interior are ten vertical plates extending from the margin of the test 

 towards the mouth, and placed one on each side of the ambulacral 

 areas. Cretaceous. Ex. D. cylindrical Chalk. 



Pygaster. Test large, depressed, outline pentagonal or 

 circular, base concave. Apical disc small ; madreporic plate large, 

 extending to the front of the periproct. Ambulacra straight, 

 simple ; pores unigeminal ; tubercles in vertical rows. Interam- 

 bulacra wide, tubercles perforate. Peristome central, large, 

 decagonal. Periproct very large, placed just behind the apical disc. 

 Inferior Oolite to Cretaceous. Ex. P. umbrella, Corallian. 



SUB-ORDER 2. ATELOSTOMATA 



No jaws or perignathic girdle. The bilateral symmetry 

 of the test is particularly well-marked. 



Hyboclypeus. Test oval, depressed, anterior part usually 

 w. p. 9 



