6 PROCEEDINGS OF THE NATIONAL MUSEUM vol.80 



the primary spines are circular, subtriangular, or oval in cross sec- 

 tion, tapering, with parallel sides, or with slightly divergent sides, 

 becoming broad and much flattened below the ambitus, much longer 

 at the ambitus than elsewhere; the miliary spines are short, rather 

 stout, mostly tapering and abruptly truncated at the tip, the smaller 

 slender and club shaped ; and the primary tubercles are of moderate 

 size, one to each interambulacral plate above the ambitus, two, and 

 later two larger and one smaller at and below the ambitus. 



Genotype. — Zenocentrotus kellersi^ new species. 



Range. — Only known from Niuafoou Island, between Samoa and 

 Fiji. 



Included species. — Zenocentrotus kellersi and Z. paradoxus. 



Remarks. — Except for the position of the longer axis, the test of 

 Zenocentrotus., especially of Z. paradoxus., is very similar to that of 

 Echinometra vanbrunti, in which the test is much flattened, the 

 longer axis is often only slightly longer than the shorter, and there 

 are numerous pore pairs — usually 7 or 8. In a specimen of Echino- 

 metra vanbrunti of approximately the same size as the type speci- 

 men of Zenocentrotus kellersi^ there are 15 interambulacral and 20 

 ambulacral plates in each column as in that species. 



The genus Zenocentrotus., however, appears to be most closely re- 

 lated to Heterocentrotus with which it agrees in the main features 

 of the test, particularly the number and relative size of the primary 

 tubercles, and the shape and structure of the poriferous areas on the 

 oral surface. The test is, however, much more nearly circular and 

 much more depressed than in Heterocentrotus. Though much more 

 slender, the primary spines are of the same general character as 

 those of Heterocentrotus trigonarius., and the young specimens of 

 this species have a very considerable general resemblance to the 

 species of Zenocentrotus., some to Z. kellersi (compare pi. 8, fig. 1, 

 and pi. 3, fig. 1), and others to Z. paradoxus (compare pi, 8, fig. 2, 

 and pi. 6, fig. 2). In a specimen of Heterocentrotus trigonarius with 

 the test 80 mm. in the longer axis, (here are 10 or 11 interambulacral 

 and 14 to 15 ambulacral plates in each column, the plates being there- 

 fore considerably fewer than in Zenocenti^otus. 



The longer axis in Heterocentrotus passes through ambulacrum 

 IVb and interambulacrum lb, while in Podophora and in Coloho- 

 centrotus it passes through interambulacrum 4a and ambulacrum Ila 

 as in Zenocentrotus. As in Echinometra., the test in Podophora and 

 in Colohocentrotus varies from subcircular or rounded pentagonal 

 to elliptical, and from much flattened to rather strongly arched. 

 The broad poriferous zones, especially the greatly broadened porifer- 

 ous areas on the oral surface, the very numerous tubercles on the 

 interambulacral plates, and the more or less marked concavity of the 



