118 ECHINODERMA. ECHINOIDEA 



In most of the regular echinoids the apical disc is large, 

 but particularly so in Gidaris, Salenia, Peltastes, and 

 their allies. In a few regular forms (fig. 47 D) the genital 

 plates are completely separated from one another by the 

 oculars, so that a single row of ten plates encircles the 

 periproct. Each genital plate has usually one perforation 

 only, but in many Palaeozoic forms (fig. 47 D) there are 

 three or more, and in Gidaris often two. Similarly the 

 oculars in some Palaeozoic echinoids have two perforations 

 instead of one. In Salenia and Peltastes there is an extra 

 plate in the apical^disc ; it is in front of the periproct and 

 is known as the sur-anal plate (fig. 47 A, b). 



In the irregular echinoids the apical disc is small, since 

 it does not enclose the periproct. The madreporic plate 

 extends to the centre of the disc (fig. 47 E, m), and some- 

 times (Spatangas) reaches to the posterior border, separating 

 the posterior oculars. The posterior genital is sometimes 

 absent (fig. 47 B), and when present may be without 

 a perforation (fig. 47 E). In Echinocorys and Holaster 

 the apical disc is elongated, and the anterior genitals are 

 separated from the posterior genitals by two oculars which 

 join in the middle (fig. 47 B) ; in Gollyrites (C) the apical 

 disc is still more elongated, since the two posterior oculars 

 are separated from the rest of the apical disc by a chain 

 of small plates. 



The corona consists of twenty columns of plates, each 

 column extending from the apical disc to near the mouth. 

 The plates are of two kinds, ambulacral (fig. 46 A, a) and 

 inter ambidacral (b) ; they are arranged in pairs, there 

 being five double columns of ambulacrals separated by five 

 double columns of interambulacrals ; each double column 

 is termed an area. The former end against the ocular 

 plates, the latter against the genital, and in each case 



