VIII 



ECHINODERMA T A SYSTEM A TIC RE VIE W 



289 



present. Anal area either within the apical system, or outside it, in the posterior 

 interambulacral area. Palaeozoic forms. 



Order 1. Bothriocidaroida. 



Regular Echinoidea, with a more or less spherical, firm test. In each inter- 

 radius there is only one meridional row of plates ; in each ambulacral area there are 

 two. Anal area, with anus within the apical system. Mouth in the centre of the 

 oral surface. Bothriocidaris. 



Order 2. Perischoechinoida. 



Regular Echinoidea. More than two meridional rows of plates in each inter- 

 radius. Two or many meridional rows in each radius. Test thick and rigid, or 



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FIG. 230. Palseechinus elegans M'Coy 

 (after Baily). 



FIG. 231. Tiarechinus princeps Laube (after Loven). 

 1, Genital aperture ; 2, anus; 3, basal; 4, radial; 5, ambu- 

 lacrum ; 6, the 3 upper plates of an interambulacrum. 



thin ; in this latter case more or less imbricated. Jaws present. Fam. Archseo- 

 cidaridae : Lepidocentrus, Archceocidaris ( = Echinocrinus), Palceechinus (Fig. 230) 

 Fam. Melonitidse : Melonites. 



Order 3. Plesiocidaroida. 



Test small and rigid, almost hemispherical. Apical system very large, with 

 large united basal plates and central anal area. Ambulacra narrow, with two meri- 

 dional or vertical rows of plates. Interambulacra with one single peristome plate, 

 followed by three plates separated by vertical sutures. Tiarechinus (Fig. 231). 



Order 4. Cystocidaroida. 



Test irregular (exocyclic), spherical or ovoid, thin and flexible. Madreporite 

 central. Ambulacral areas narrow, with two vertical rows of plates. Interambu- 

 lacral areas broad, with numerous vertical rows of scale-like movable plates. Anus 

 in the posterior interambulacrum above the ambitus. Echinocystis ( = Cystocidaris}. 

 VOL. II U 



