PRIONECHINUS SCULPTUS. 307 



well arched and only slightly flattened abactinally. The sculpturing is confined 

 to the oculo-genital ring and there is great diversity in the degree to which it is 

 developed there. The whole test is well covered with granules and tubercles 

 and the median interambulacral suture is sufficiently depressed to be rather 

 conspicuous. On the ocular and genital plates there are not only numerous 

 small tubercles but irregular or radiating furrows and low ridges. In some speci- 

 mens these are almost wholly wanting while in others, they are so conspicuous 

 (under a lens) as to give the whole abactinal system a highly ornamented ap- 

 pearance. The genital plates (PI. 101, fig. 6) are moderate and subequal, the 

 madreporite, with its minute group of a very few water-pores, being distinguish- 

 able only with difficulty. The genital pore is near the centre of each plate. The 

 ocular plates are rather large, but are all broadly exsert. The periproct is 

 covered by a considerable number of small, rounded plates but among them the 

 suranal is easily distinguishable. The ambulacra are rather narrow, little 

 more than half as wide at ambitus as the interambulacra. The poriferous areas 

 are narrow but rather conspicuous, as the pores are large; the pore-pairs are 

 arranged in nearly vertical arcs of three. The actinostomal membrane (PI. 101, 

 fig. 5) is very thin and distal to the buccal plates is perfectly naked. Within 

 their circle, there are small plates, sometimes abundant enough to cover that 

 part of the membrane very fully but in other cases imperfect and scattered. 

 The buccal plates themselves are of moderate size and all carry normal tube- 

 feet ; the two plates of each pair are very close together, but the pairs are well 

 separated from each other. The gill-cuts are insignificant. The spines are 

 rather thick and blunt or bluntly pointed. The primaries have about 10 longi- 

 tudinal striations. 



The pedicellarise are abundant and not specially characteristic, as they are 

 very similar to those of Chuni. While the globiferous (PI. 93, fig. 24) and ophi- 

 cephalous are common enough, only two tridentates and no triphyllous were 

 found. The tridentate have very narrow, compressed valves (PI. 93, fig. 25) 

 about .60 mm. long, meeting only at the tip where they are somewhat expanded 

 and have minutely serrate margins. 



The color of the test and smaller spines is dull purplish red, often very light 

 and becoming nearly white in some specimens. The primaries are white, but 

 the longitudinal striations are purplish. As a result of this coloration, the 

 general aspect of the specimens is rather diverse, the exact shade ranging from 

 distinct, though dull, purplish red to almost white. 



This is a well-marked species and it is not likely to be confused with any 



