80 ECHINODERMATA. ECHINOIDEA. 



tubercles, which are imperforate, and may be crenulate. 

 Peristome slightly notched. Lower Greensand to Gault. 



Hemicidaris. Test spheroidal, inferior surface flat- 

 tened. Apical disc small. Ambulacral areas narrow, 

 slightly flexuous, with two rows of tubercles on the under 

 surface, which pass into granules on the upper. Pores 

 unigeminal, but bigeminal or trigeminal near the mouth. 

 Interambulacral areas broad ; plates large and few, each 

 with a large perforated and crenulated tubercle, and also 

 smaller tubercles and granules. Spines cylindrical, long. 

 Peristome large, decagonal. Inferior Oolite to Purbeck 

 Beds. 



(6) Irregular Euechinoids. 



The test is bilaterally symmetrical. The anus is 

 placed outside the apical disc. The mouth is central or 

 excentral. Ambulacra simple or petaloid. Jaws may be 

 present or absent. 



Echinoconus (= Galerites). Test conical, or almost 

 hemispherical, inferior surface flat, outline pentagonal or 

 oval. Apical disc small, with four genital plates. Ambu- 

 lacral areas, narrow, straight, simple ; pores bigeminal, 

 trigeminal near the mouth. Interambulacral areas with 

 broad plates, tubercles very small, perforated and crenu- 

 lated. Peristome small, central, decagonal. Jaws absent. 

 Periproct marginal or submarginal. Upper Greensand to 

 Upper Chalk. 



Holectypus. Test hemispherical, depressed, base 

 excavated. Apical disc small. Ambulacral areas narrow, 

 straight, simple ; pores unigeminal, tubercles small. In- 

 terambulacral areas with rather large plates, tubercles 

 small. Peristome central, decagonal. Jaws present. 



