ARBACIA NIGRA. 401 



Arbacia nigra 



Echinus niger (Molin.), 1782, Chili. 



! Arbacia nigra Gray, 1835, Proc. Zodl. Soc. London. 



PL I", f. 6,7. 



Abactinal system remarkable for the great size of the anal system, sur- 

 rounded by small, triangular genital plates, three of which are separated by 

 the ocular plates, reaching to the anal system. The genital plate carrying 

 madreporic body is much larger than the others, though with younger speci- 

 mens this difference in size is not so marked ;. genital openings large ; ocular 

 plates small, often mere knobs in older specimens; granulation of abactinal 

 system forming radiating lines over the anal and the genital plates. Inter- 

 ambulacral coronal plates much higher nearer abactinal system than towards 

 ambitus. Above ambitus coronal plates carry one large tubercle, close to 

 the poriferous zone, from which extends a horizontal row, parallel with the 

 suture of plates of as many as four smaller tubercles, diminishing, according 

 to their position, towards the median line, placed near the lower part of the 

 plate. The upper part of the plate is filled with secondary tubercles of 

 nearly uniform size, irregularly arranged, the whole space between them 

 filled by miliaries carrying pedicellariaa. The secondary tubercles are fre- 

 quently united, leaving the interambulacral median space above ambitus more 

 or less bare, covered simply by granulation of miliaries. The narrow coronal 

 plates from ambitus to actinostome carry one row of large primary tubercles 

 of uniform size, closely packed together, forming diagonal transverse rows, 

 and, in connection with the horizontal rows of the upper part of test, more 

 or less apparent vertical rows. The horizontal rows of the primary tubercles 

 of the actinal part of test are separated by miliaries. There are two well- 

 defined vertical rows of primary tubercles, of nearly uniform size, in the 

 median ambulacral space, surrounded by a ring of miliaries. Tbese are 

 flanked by a broad poriferous zone, the pores being crowded laterally at an 

 early age (35 mm , diam.), so that, in large specimens, the pores appear ar- 

 ranged in an irregular, zigzag manner. Owing to this lateral crowding of 

 the pores, the poriferous zone is extremely petaloid round the actinostome, 

 the interambulacral space being reduced to a very narrow point between 

 the prolongation of the prominent lip of the actinal cuts. 



Test regularly arched ; outline more or less circular ; the test frequently 

 more or less gibbous, especially near ambitus in the interambulacral zone. The 

 spines of the upper part of test, in median interambulacral space, are quite 



