THE GENUS COLOBOCENTROTUS. 21 



in P.atrata (Pis. 20, fig 2; 21, fig. i), while it is quite narrow in P. pedifera 

 (Pis. 8, fig. 1; 9, fig. l). In the latter there are two rows of small primary 

 tubercles, while there are four in the former. 



Figures ^ of Plates 9 and 21 show how large an area of the actinal surface is 

 occupied by the closely packed ambulacral suckers, and we can judge of the great 

 power exerted by these numerous suckers in keeping hold of the rock to which 

 they are attached even when exposed to the full action of the surf on the outer 

 beach of a coral reef. The sea has practically no hold on the pavement of 

 radicles which covers the abactinal side of the test, and thus with the ambu- 

 lacral suckers a cup is formed which it is difficult to detach from the rocks. 



The large and small primary tubercles of P. pedifera are comparatively 

 larger than those of P. atrata ; the former occupy nearly the whole of the inter- 

 ambulacral and ambulacral plates leaving but little space for the smaller 

 secondaries and miliaries, while in P. atrata the tubercles leave quite a part of 

 the plate bare which carries a few miliaries and small secondary tubercles, and 

 is covered by an indistinct granulation. In both these species the tubercles 

 are most prominent, very glossy as if they were rounded beads made of wax 

 (Pis. 8, 20, figs. 4, 6). 



The tubercles diminish somewhat in size from the ambitus towards the 

 abactinal system (Pis. 8, fig. 3; 9, fig. 3; 20, fig. 6; 21, fig. 3) which is covered 

 by small secondary tubercles, with the exception of the anal system ; but even 

 that is completely hidden by the pavement of secondary radioles on the geni- 

 tal plates which lap over the anal system and conceal it (Pis. 7, fig. 2; 20, 

 fig. 3, and Pis. 9, fig. 5; 21, fig. 5). The genitals carry a few miliaries. 

 In both these species the anal system is covered with two or three irregularly 

 concentric rows of small triangular plates (Pis. 9, fig. 2; 21, fig. 2). Accord- 

 ing to the size of the specimens there are in P. pedifera from six to eight large 

 tubercles at the ambitus in the interambulacral areas forming six to eight 

 vertical rows, of which only two extend to the actinal system. There are 

 but two vertical rows of primary tubercles in the ambulacral zones (PI. 9, 

 fig. 3), with a few small miliaries along the median line. Towards the ab- 

 actinal system a few small secondary tubercles occupy a part of the outer 

 edges of the interambulacral plates. On the actinal side towards the actino- 

 stome the primary tubercles of the ambitus are soon replaced by small 

 secondaries both in the ambulacral and interambulacral areas (Pis. 8, figs. 

 1, 3; 9, figs. 1, 3; 20, fig. 2; 21, fig. l). 



A comparison of the left anterior interambulacrum of a denuded specimen 



