390 PHYLLACANTHUS GIGANTEA. 



Median ambulacra! zone narrow, with two exterior rows of coarse tubercles 

 and two median irregular rows of smaller alternating tubercles. The nar- 

 rowness of the median ambulacral field distinguishes this species from P. 

 imperialis, as well as the coarser character of the miliaries filling the median 

 interambulacral space. In the smallest specimens examined of P. imperialis, 

 the greater width of the median ambulacral zone, the number of vertical 

 rows of miliary tubercles in the median interambulacral space, and the nature 

 of the granulation were already well marked, as differing from P. dubia; so 

 that, in consequence of the coarser granulation of the secondary and miliary 

 tubercles, the ambulacral and interambulacral papillae which they carry are 

 broader, as well as those surrounding the hase of the primary spines. 



This species, when not denuded, can apparently he readily distinguished 

 from its allied congener even when covered with its spines. The primary 

 spines are nearly cylindrical, sometimes slightly swelling near hase, or gently 

 tapering towards the extremity ; they are deeply grooved for their whole 

 length. The granulation forming the lamellae is extremely compact, the 

 whole interlamellular space is extremely porous ; while in P. imperialis the 

 granulation is most distinct, and the texture of the cylindrical or bat-shaped 

 spine homogeneous and solid. 



The color of the secondary spines and papillae is violet ; the porous part of 

 shaft of spine is grayish, with darker-colored longitudinal lamellae of violet 

 or bluish tinge. The base of spine is reddish-brown, the milled ring promi- 

 nent; the spine tapers from the milled ring to the commencement of the 

 fluting, the line of demarcation between the shaft of the spine and annular 

 ring at hase of spine, above nulled ring, heing very marked. 



Zanzibar; Bunin Islands ; Australia. 



Phyllacanthus gigantea 



! Chondrocidaris gigantea A. Ac, 180.3, Bull. M. ('. Z. 



\ Phyllacanthus gigantea A. Ac, 1872, Rev. Ecli., l't. I. p. 150. 



PI. I'; PI. P. f. 27-.;i. 



Only three large specimens of this species have been accessible to me. 

 The primary interambulacral tubercles are ten in number, the greatest num- 

 ber thus far noticed in any species of Phyllacanthus and Cidaris proper. 

 The scrobicular circle is elongated, the median interambulacral space covered 

 by miliaries is more than half the total breadth of the interambulacral zones. 

 The miliaries are densely packed, of uniform size, and carrying very short 



