ART. 5 ECHINODERMS FROM THE TONGA ARCHIPELAGO CLARK 7 



oral surface of the test along the longer axis, readily separate Podo- 

 phora and Colohocentrotius from Zenocentrotus. The teeth of Zeno- 

 centrotus, it may be noticed, with their strongly beveled sides and 

 broad flat upper surface bordered with fine rounded ridges, re- 

 semble the teeth of Podophora more closely than they do the teeth 

 of the other genera. 



The tube-feet resemble those of related genera, but the sucking 

 disk is divided into almost invariably 5 (occasionally 6 and rarely 

 4) sectors, instead of into 4 as in Podophora and Colohocentrotus or 

 5 or 6 in Heterocentrotus. 



KEY TO THE SPECIES OF THE GENUS ZENOCENTROTUS 



a\ Poriferous areas below much broadened, in the widest portion 

 three-fourths the width of the adjacent portion of the inter- 

 ambulacral area, with the outer border strongly convex; 

 tubercles on the interambulacral plates above the ambitus 

 with their bases separated by less than the diameter of the 

 tubercles ; primary spines at ambitus long and mostly some- 

 what tapering, the spines gradually decreasing in length 

 toward the pei*iproct kellersi. 



a^ Poriferous areas below not greatly broadened, in the widest 

 portion not more than half the width of the adjacent por- 

 tion of the interambulacral area, with the outer border gently 

 convex ; tubercles on the interambulacral plates above the 

 ambitus with their bases separated by much more than the 

 diameter of the tubercles ; spines at the ambitus less elongated 

 and mostly increasing slightly in width distally paradoxus. 



ZENOCENTROTUS KELLEKSI. new species 



Plates 1-3 ; Plate 4, Figure 1 ; Plate 5, Figure 1 ; Plate 6, Figure 3 



Description of the type specimen. — The longer diameter of the 

 test is 55 mm., and the shorter diameter is 47.5 mm. ; the height is 

 19 mm. 



There are 15 or 16 interambulacral and 20 ambulacral plates in 

 each column. 



The periproct (pi. 1) is irregularly oval, 4 by 3 mm.; it bears 

 nmnerous small plates, which decrease in size inwardly; a very few 

 of the larger plates bear tubercles. The madreporic plate is more 

 than half again as large as the other genitals, which are subequal 

 in size. It is situated in the middle half of one of the sides of the 

 periproct delimited by the longer axis. It bears about 14 very 

 minute tubercles, which are arranged roughly in two irregular trans- 

 verse rows across the middle. The other genitals are subequal in 

 size. Each bears 7 to 9 small tubercles, of which 2, situated on the 

 inner margin, are much larger than the others. 



