IQ PROCEEDINGS OF THE NATIONAL MUSEUM vol.80 



In the smallest specimen (pi. 6, fig. 3) the diameter of the test is 

 25.5 mm. The longest spines at the ambitus are 20 mm. long and 

 the spines nearest the periproct are 12 mm. long. The color is dull 

 olive. 



ZENOCENTROTUS PARADOXUS, new species 



Plate 5, Figuee; 2 ; Plate 6, Figures 1, 2 ; Plate 7, Figures 1, 2 



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

 test is 37 mm., and the shorter diameter is 33 mm.; the height is 

 13.5 mm. 



There are 13 or 14 interambulacral and 16 ambulacral plates in 

 each column. 



The large spines at the ambitus (pi. 5, fig. 2 ; pi. 6, figs. 1, 2) are flat- 

 ter and more broadened than in Z. kellersi, all of them increasing 

 slightly in width distally or having parallel sides ; none taper distally. 

 On the apical surface the spines very rapidly decrease in size above the 

 ambitus, almost immediately becoming short, and mostly very short, 

 the longest not more than 7 or 8 mm. in length. About the peristome 

 (pi. 7, fig. 2) there is in each interambulacral area a group of usually 

 4 long, slender, narrowly spatulate spines 4-5 mm. long which stand 

 out rather conspicuously because of their length from the other 

 spines on the oral surface. In Z. kellersi (pi. 4, fig. 1) the spines 

 in this position are not noticeably different from those behind them. 



The oculars (pi. 6, fig. 1) are larger than in the type specimen of 

 Z. kellersi, with I and V more fully exsert; each bears from 1 to 3, 

 usually 3, very small tubercles. 



The tubercles on the interambulacral and ambulacral plates are 

 relatively smaller than in Z. kellersi, and are more widely separated. 



There are 10 or 11, usually 11, pore pairs in an arc. 



On the oral surface (pi. 7, fig. 1) the poriferous zones are less 

 broadened, being where they are widest about half as broad as the 

 interambulacral areas separating them. Their outer margin is much 

 less strongly curved than in Z. kellersi, and the tubercles in the two 

 ambulacral rows between the poriferous zones are smaller at the 

 ambitus and decrease more gradually in size. 



Type.— U.S. l^M. No. E. 2813, collected at Niuafoou by Lieut. H. C. 

 Kellers on September 20, 1930. 



Thirteen additional specimens were collected on September 19 and 

 20 and on October 6, 1930. 



HETEROCENTROTUS TRIGONARIUS (Lamarck) 



Plate 4, Figubb 2; Plate 8 



Twenty-seven specimens. In the largest specimen the longer diam- 

 eter of the test is 55 mm., and the longest spines are 75 mm. long. 



