12 TROPICAL TACIFIC ECHINI. 



rior ambulacrum. On fig. 4 of the same plate are shown the corresponding 

 vertical rows of the right anterior ambulacrum and interambulacrum. In 

 both the interambulacra two of the vertical rows along the median line are 

 included by the inner and outer rows and do not extend either to the abactinal 

 or the ambital region. 



The abactinal system when denuded shows that the genital plates are 

 covered with closely packed primary tubercles only slightly smaller than those 

 of the rest of the abactinal surface of the test above the ambitus. (Pis. 34, 

 fig. S; 38, fig. 2; 43, fig. S.) There are in addition a few miliaries and small 

 secondaries irregularly scattered between the larger tubercles. 



The madreporic genital is much larger than the other genitals ; the madre- 

 porite occupies the greater of the two sides of the plate. The proximal part 

 carries two primary tubercles and from four to five smaller ones as well as 

 a few miliaries. The genital openings are large, slightly sunken ; the lateral 

 posterior genitals are smaller than the odd posterior genital and the left ante- 

 rior ones. (PI. 43, fig. S.) 



The genital plates are heptagonal. The left anterior genital carries no less 

 than sixteen primaries with a few miliaries and a couple of smaller primaries. 

 (PI. 41, fig. 7.) The other genitals carry from twelve to fourteen primaries 

 with a few miliaries and secondaries. 



The ocular plates are pentagonal, they carry three primary tubercles with 

 two or three small miliaries, the ocular pores are very small. The posterior 

 oculars approach the anal system nearer than the anterior pair or the odd 

 ocular. (PI. 43, figs. S, 3.) 



The anal system is irregularly circular and covered with three to four con- 

 centric rows of irregularly shaped pointed and elliptical plates. (PL 43, figs. 

 2,3.) 



In a specimen of Colob. Mertmm. measuring 88 mm., when seen from 

 above the primary radioles covering the abactinal system are seen to be some- 

 what smaller than those of the rest of the test (Pis. 33, fig. 2; 37, fig. 2; 43, 

 fig. 1), with the exception of a few larger triangular and elliptical ones which 

 bridge over the anal system (Pis. 40, figs. 30-32; 43, fig. l). 



On Plates 33 and 37 are given views of a specimen of Colob. Mertmsii,'zAZ^/^^y^ 

 measuring 88 mm. in diameter. The ambital fringe of large truncated flat- j 



tened primary spines carried by the larger tubercles on the actinal side of 

 the ambitus and at the ambitus resemble slightly those of Podophom atrata ; 

 they are comparatively shorter and narrower (Pis. 33, figs. 1, 2; 37, figs. 1, 2) 



