ECHINODERMATA 53 1 



Others were probably immature. In the one specimen, they were very 

 small but seemed to be present only on the left side of the mesentery. 

 Calcareous deposits, very regular tables, the bases perforated by a 

 central, and 8 or 9 peripheral holes, the rather low spires surmounted 

 by 20-30 teeth ; underneath the layer of tables are numerous buttons, 

 most of which are not symmetrical, and have 8-12 holes, though many 

 have only six ; in and about the pedicels the buttons increase in size 

 and gradually become transformed into curved supporting plates with 

 four or more longitudinal series of holes ; in the papillcE, are curved, 

 rough supporting rods ; the holes in the buttons and plates are not 

 large nor of uniform size, but are quite irregular. The tables are 

 very closely crowded in the outer layer of the body wall so that they 

 form a rough, crisp layer, though very delicate, over the whole sur- 

 face ; it is because of this that the name frequentiamensis has been 

 selected. Color light brown, with a distinctly reddish tinge ; the ten- 

 tacles and pedicels darker. Length of largest specimen about 40 mm. 

 Although this species is undoubtedly closely related to captiva and 

 resembles it in some respects, it is distinguished from that species by 

 the shape and arrangement of the calcareous particles, especially the 

 buttons. In these respects also it differs from difficilis although it 

 may be even nearer this latter species. Semper's description is very 

 brief and he gives the figure of only one table and two buttons, so it is 

 possible the Clipperton Island specimens are really nearer those from 

 Samoa than would appear from the descriptions. But they differ in 

 size, color and number of Polian vessels, as well as in the calcareous 

 particles. Moreover, the genital gland is attached to the dorsal mesen- 

 tery near the middle of the body in frequentiamensis^ and not close 

 behind the calcareous ring as Semper says it is in difficilis. 



