78 ECHINODERMATA. ECHINOIDEA. 



arches, known as auricles, placed over the ambulacra just 

 within the peristome. 



In the irregular echinoids there is a well-marked 

 bilateral symmetry; a plane which passes through the anus 

 (which is posterior), the apical disc, and the mouth, will 

 divide the body into two similar parts. The ambulacra are 

 often markedly dissimilar, the anterior one differing from 

 the others which are paired. The bilaterality is quite 

 inconspicuous 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. 



OEDER. PAL2EECHINOLDEA. 



The number of rows of plates forming the test varies. 

 In each interambulacral area there is one, or more than 

 two ; in each ambulacral two or many. The plates 

 frequently overlap, giving the test a certain amount of 

 flexibility. All the genera, except Echinocystites, are 

 regular. In the genital plates there are from three to 

 five pores, in the oculars two or none. Jaws are present. 



Palaeechinus. Test spheroidal, rigid. Apical disc 

 central, enclosing the anus. Five large genital plates, 

 each with three perforations, one may have one only. 

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

 Ambulacral areas narrow, with two rows of plates, pores 

 unigeminal. Interambulacral areas wide, with four to 

 eight 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. 



