148 HAWAIIAN AND OTHER PACIFIC ECHINI. 



primary element is the outermost of the three, that of the adoral sec- 

 ondary is innermost, while that of the aboral secondary is intermediate ; 

 we thus get the characteristic arrangement of the pore-pairs in arcs of 

 three. In Astropyga, only the alternate primary elements are like each 

 other in form, one series being broadest at the inner end, and those 

 alternating with them broadest at the middle ; the secondary elem^ents 

 accompanying the latter occupy the same position they do in Echino- 

 thrix, while those whicli accompany the plates with a wide inner end 

 lie close to that end ; the pore-pairs of the primary elements are here 

 median in position, while the pore-pairs of the secondary elements alter- 

 nate, two pairs being inner, then two outer, then two inner, etc. The 

 characteristic arcs of three are thus maintained. In Micropyga the ele- 

 ments are arranged essentially as in Echinothrix, but the pore-pairs of 

 the primaries alternate in position, as well as those of the secondaries 

 (as in Astropyga) ; thus, if the pore-pair of a given primary element lies 

 at the outer end of the plate, the pore-pairs of the secondary elements of 

 the same plate occupy a median position, while in the adjoining plates 

 the pore-pairs of the primary elements will be median in position and 

 those of the secondary elements take an outside position ; we thus get 

 the double colunm of pore-pairs characteristic of Micropyga. In most of 

 the Echinothuridae the condition is essentially as in Echinothrix, but the 

 secondary plate elements remain so small and are so generally pushed out 

 of position, the appearance of an ambulacrum is quite different from that 

 of any Diadematid ; the abactinal arrangement of the pore-pairs is com- 

 monly as in Echinothrix. In Sperosoma, the secondary plate elements 

 become so large actinally, they separate the inner half of the primary ele- 

 ment from the cuter, and there thus appear to be four columns of plates 

 in each half-ambulacrum. Mortensen says, in his diagnosis of Sperosoma, 

 " The secondary ambulacral plates on the actinal side of the same size as 

 the primary ones." We do not find this to be the case in the type- 

 species, Grimaldii, nor in any other of the species we have examined ; it 

 seems to us that the halves of the primary element are, with rare excep- 

 tions, noticeably larger than the secondary elements. 



The difference between the Echinothuridcc and Diadematida3 in the 

 imbrication of the coronal plates is one of degree and not of kind, and 

 the same is true of the presence of membranous interspaces between the 

 plates. The difference between the test of an Echinothrix or Centroste- 



