ECHINODERMA. ECHINOIDEA 125 



in the regular sea-urchins, but the plane of symmetry may 

 be found by means of the madreporic plate, which is 

 always placed at the upper end of the right anterior 

 interambulacral area. 



The Echinoidea may be divided into two Orders, 

 (1) Regularia, (2) Irregularia. 



ORDER I. REGULARIA 



The mouth is at the centre of the base, and the anus 

 within the apical disc. The ambulacra are simple. Jaws 

 are present in all. The test is circular in outline. 



Palaeechinus (fig. 47 D). Test spheroidal, rigid. Apical 

 disc with five large genital plates, each with three perforations, — 

 but in some cases one of the genitals may have a single perforation. 

 Ocular plates five, small, each with two perforations. Ambulacra 

 narrow, straight, with two rows of j^lates, and smaller plates 

 at the margins ; two vertical rows of pairs of pores on each side 

 of the area. Interambulacra wide, with five to seven rows of 

 plates at the equator, fewer towards the poles; plates hexagonal, 

 except those next the ambulacral area, which are pentagonal ; surface 

 of plates covered with granules. Spines small. Upper Silurian to 

 Carboniferous Limestone. Ex. P. intermedins, Carboniferous. 



Melonechinus ( = Melonites) (fig. 51). Test large, elliptical, 

 with furrows from apex to peristome. Apical disc with five genital 

 plates, each having from two to five pores. Ambulacra broad, 

 concave on each side of a median ridge, with five to fourteen columns 

 of plates, each plate with a pair of pores ; four plates at the peri- 

 stomal edge of each area. Interambulacra consisting of four to 

 eleven columns of small thick plates, which are pentagonal next the 

 ambulacra, hexagonal elsewhere ; tubercles very small. Jaws large. 

 Carboniferous. Ex. M. multiporus. 



Archaeocidaris ( = Echinocrinus). Test large, plates over- 

 lapping. Ambulacra narrow, formed of two rows of plates ; pores 

 unigeminal. Interambulacra consist of from four to seven rows of 

 large plates, the middle ones being hexagonal ; each plate has 



