ECHIXODERMA. ECHINOIDEA 127 



margin. Cretaceous to present day. Ex. 8. petalifera, Upper 

 Greensand. 



Acrosalenia. Form similar to Peltastes. Apical disc rather 

 large ; genital plates large, the posterior smaller than the others and 

 differing in shape. One or more sur-anal plates in front of the 

 periproct, which is in the antero-posterior line. Pores unigeminal 

 except near the peristome. Interambulacra with large perforate 

 tubercles. Spines smooth or striated. Lias to Cretaceous. Ex. 

 A. spinosa, Inferior Oolite to Corallian. 



Hemicidaris. Test spheroidal, inferior surface flattened. 

 Apical disc small. Ambulacra narrow on the upper surface, slightly 

 flexuous, with two rows of tubercles below the equator, which pass 

 into granules on the upper surface ; plates below the equator com- 

 pound, each formed of three fused plates. Pores unigeminal, but 

 bigeminal near the peristome. Interambulacra broad ; plates large 

 and few, each with a large perforate and crenulate tubercle, and 

 also smaller tubercles and granules. Spines cylindrical, long. 

 Peristome large, decagonal. Inferior Oolite to Cretaceous. Ex. H. 

 intermedia, Corallian. 



Pseudodiadema (fig. 48 A). Test circular or slightly poly- 

 gonal, depressed. Apical disc and periproct large. Ambulacra 

 straight, narrower than the interambulacra, with two rows of crenu- 

 late and perforate tubercles ; plates compound, each consisting of 

 three fused primaries, the middle being largest, usually with three 

 pairs of pores on each plate, unigeminal. Interambulacra with two 

 or more rows of primary crenulate and perforate tubercles. Peri- 

 stome large, decagonal. Lias to Tertiary. Ex. P. ornatum, Lower 

 Chalk. 



Hemipedina. Test circular or slightly polygonal, depressed. 

 Apical disc rather large. Ambulacra narrow, plates formed of three 

 fused primaries (but simple near the apical disc), pores unigeminal ; 

 two rows of tubercles, perforate, not crenulate. Interambulacra 

 with two (sometimes more) vertical rows of primary, perforate, not 

 crenulate tubercles. Spines of moderate length, finely striated. 

 Peristome with slight incisions. Lias to present day. Ex. H. ethe- 

 ridgei, Lias. 



