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BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



interambulacral plate. The pores are half vertically placed. The 

 peristome (fig. 6) is especially interesting; it is naked except for five 

 large buccal plates, each with two pores for the buccal tube feet, and 

 one or two ambulacra! plates, just detached from the ambulacra. 

 The large buccal plates are apparently simple, but on closer inspection 

 are seen to be divided by a fine median line into two lateral halves ; 

 thus they were originally paired, as was to be expected. As yet 

 there are no interambulacral plates on the peristome. It is 



Fig. 6.— Peristome of young specimen of Histocidaris elegans, of 7 mm 

 diameter. x20 



important to notice that there is no trace of an unpaired primary 

 interambulacral plate. 



The apical system (fig. 7.) is very simple, perfectly regidar, with 

 very few tubercles and, of course, still without genital pores. The 

 longest spines are 27 mm. in length, nearly four times the diam- 

 eter of the test. The first 1 or 2 oral primaries already have their 

 typical strongly serrate form. The pedicellariae are still only of the 

 smaller form. 



The character of the apical system especially shows that the spec- 

 imens described as " Cidaris (Histocidaris) elegans, juv.?" by de 

 Meijere 2 can hardly belong to the same species as the young spec- 



iSiboga Echinoidea, p. 25, pi. 2, fig. 17; pi. 12. figs. 136-140. 



