180 SYNOPTIC COLLECTION. 



the ocular plates join the membranous disc, and separate 

 the genital plates, while the other two which are each side 

 of the madreporic plate are crowded outside of the ring so 

 that they do not touch the anal membrane. 



The slender spines of the adult (No. 325) are usually 

 more or less solid though in youth they are hollow. They 

 vary in size, the smaller ones being light colored and the 

 large ones dark with longitudinal ridges. These ridges 

 are provided with short pointed teeth, so that one cannot 

 pass the finger downward from the tip end of the spine to 

 its base without being pricked by the sharp points. 



Echinothrix turcarum Ret. (No. 327), has the pores 

 in arcs of three pairs similar to those of Diadema, but 

 unlike this genus the arcs are independent of each other. 

 The ambulacra broaden out slightly on the abactinal sur- 

 face, suggesting the petaloid condition of the more special- 

 ized Clypeastroids. 



The ambulacral areas are crowded with many small 

 spines, while the longer and more delicate ones are on the 

 interambulacra. 



Colobocentrotus atratus Br. (No. 328), is peculiarly 

 modified in the shape of its corona and spines. The ven- 

 tral side of the former is very flat and the dorsal part rises 

 like a low dome. The closely set, dark colored spines, 

 like tiles in a pavement, cover this dome, completely con- 

 cealing everything beneath. If these spines are removed 

 the low rounded tubercles are seen (No. 328). The ven- 

 tral surface is covered with short cylindrical spines 

 crowded closely together. From the ambitus the long 

 spines resembling clubs extend downward causing the sea 

 urchin to look as though it were mounted on many stilts. 

 It is these spines that mask the real shape of the corona, 

 making the dome appear much higher than it really is. 



In spite of the close pavement of spines there are many 

 tube feet in the ambulacral areas ; these are arranged in 

 arcs of six or seven pairs. 



The young Heterocentrotus mammillatus Br. (No. 329), 



