420 ASTROPYGA RADIATA. 



Astropyga radiata 



Cidaris radiata Leske, 1778. Klein, Add. 

 ! Astropyga radiata Gray, 1825, Ann. Phil. 



PI. XXIV. f. 40. 



This species, long known only from the excellent figure of Seba, is appar- 

 ently quite widely distributed in the Pacific and Indian Oceans. The 

 specimens preserved in the different collections are. however, nearly all of 

 the same size, with the exception of a small specimen from Mauritius in the 

 Museum collection. Test exceedingly depressed ; actinal surface of test 

 flattened; actinostome somewhat sunken, small, with moderately deep broad 

 actinal cuts; actinal membrane strengthened by closely packed rectangular 

 or irregularly elliptical plates covering the whole membrane; ten promi- 

 nent buccal plates. The whole actinal part of test is covered by tubercles of 

 uniform size, forming closely arranged vertical and transverse rows, one 

 transverse row upon each coronal plate. The tubercles of both areas of the 

 same size, diminishing very gradually in size towards the actinostome. In the 

 largesl specimens examined there were as many as sixteen vertical rows of 

 tubercles on the actinal surface of the test near the ambitus. Outline of test 

 seen from above is pentagonal, with rounded ambulacra extending beyond the 

 general outline. Test at ambitus rising very rapidly, regularly arched, and 

 whole abactinal pari of test Hat. the interambulacra rising but slightly towards 

 the more or less sunken abactinal system. The ambulacra rise high above the 

 interambulacra, especially near the abactinal system, when they increase some- 

 what in breadth, tapering rapidly towards the ocular plates. There are two 

 rows of primary tubercles extending to ambitus, of uniform size, somewhat 

 smaller than the primary rows of the interambulacra Between most of the 

 primaries a small secondary is intercalated in the abactinal part of the test 

 only, the rest of the plate is loosely covered by miliaries ; on the actinal sur- 

 face the primaries, being closely arranged, occupy nearly the whole plate ; the 

 secondaries and miliaries in both areas are not numerous. The poriferous 

 zone is broadest about two thirds the distance from ambitus to abactinal 

 system, tapering rapidly to apex, but growing very gradually narrower from 

 ambitus on the actinal surface, being very slightly petaloid near actinostome, 

 owing to the greater obliquity and the greater size of the pores on the 

 abactinal part of the test. 



In the interambulacral spaces the whole abactinal part of the test is occu- 

 pied by the broad bare forking band, separated from the poriferous zone by 



