128 ECHINODERMA. ECHINOIDEA 



Diplopodia. Form and tubercles similar to Pseudodiadema. 

 Pores bigeminal near the apex and peristome, unigeminal at the 

 equator. Plates at the equator composed of four primary plates or 

 sometimes the lowest plate is a demi-plate. Jurassic and Cretaceous. 

 Ex. D. versipora, Corallian. 



Stomechinus. Test hemispherical. Genital plates rela- 

 tively large, projecting outwards ; oculars small. Ambulacra wide, 

 plates formed of three primaries — the middle one largest ; pores 

 trigeminal. On each ambulacral and interambulacral area are two 

 vertical rows of primary, imperforate, non-crenulate tubercles, of 

 about the same size on each area ; also secondary tubercles and 

 granules, usually numerous. Peristome large, with ten deep in- 

 cisions. Inferior Oolite to Lower Cretaceous. Ex. S. bigranularis, 

 Inferior Oolite. 



Pbymosoma ( = Cyphosoma) (fig. 48 B). Form similar to 

 Pseudodiadema. Ambulacral plates high, compound, each may 

 consist of four, five, or six fused plates (some being demi-plates) with 

 the same number of pairs of pores ; two rows of primary imper- 

 forate tubercles ; pores unigeminal except near the apical disc. 

 Interambulacra with two or more rows of primary imperforate 

 tubercles. Jurassic to Eocene ; common in the Chalk. Ex. C. 

 koeyiigi, Upper Chalk. 



Echinus. Test more or less hemispherical. Apical disc as 

 in fig. 46 B. Ambulacra rather narrow, trigeminal, plates consisting 

 of a lower primary, a middle demi-plate, and an upper primary or 

 demi-plate. Two vertical rows of small, primary tubercles on 

 each area, and often numerous secondary tubercles. Peristome 

 rather small, circular, with small incisions. Cretaceous to present 

 day. Ex. E. ivoodivardi, Pliocene ; E. escidentus, Pliocene and 

 living. 



ORDER II. IRREGULARIA 



The anus is placed outside the apical disc, in the 

 posterior interambulacral area. The raouth is either 

 central or in front of the centre. The test is bilaterally 



