FROM THE CORAL RAG. 165 



the angles between them (fig. 2 d) ; beyond the semi-tubercles the area becomes con- 

 tracted, and the upper half of the space is filled with two marginal rows of small 

 granules from sixteen to eighteen in each row (fig. 2 c, d), the very great difference 

 between the structure of the inferior and superior parts of the ambulacral areas forms a 

 remarkable feature in the diagnosis of this urchin. 



The poriferous zones are slightly undulated (fig. 2 c, d) ; from the apical disc to the 

 fourth pair of semi-tubercles the pores are unigeminal ; but from this point to the peristome 

 the area contracts and the zones expand to fill up the increase of space at the base, in this 

 region (fig, 2 d), the pores fall into triple oblique pairs ; between the pores, forming a pair, 

 the septa form an elevated moniliform line (fig. 2 d) ; and there are five pairs of pores 

 opposite each inter-ambulacral plate. 



The inter-ambulacral areas are twice and a half the width of the ambulacral 

 (fig. 2 a, c) ; they are filled with two rows of primary tubercles, eight tubercles in each 

 row, which gradually increase in size from the peristome to the equator (fig. 2 c), and 

 then as gradually diminish as they approach the apical disc (fig. 2 a) ; the mammillary 

 bosses of these tubercles, especially those about the centre of the rows, are large and 

 prominent (fig. 2 d) ; their summits are broad and smooth (fig. 2 c, e), and the areolas 

 are not wide (fig. 2 e), there are two rows of secondary tubercles on the zonal side of the 

 primaries (fig. 2 b), which extend from the peristome to near the equator, and a few 

 others, about six, at the basal portion of the centro-sutural region (fig. 2 5, d), the miliary 

 zone is broad (fig. 2 a), and filled with from four to six rows of coarse, unequal-sized 

 granules, some of them even assuming the form of minute tubercles (fig. 2 d) ; the 

 areolas are surrounded by these granules, which form complete scrobicular circles in the 

 three uppermost tubercles (fig. 2 a), and incomplete circles in the lower ones (fig. 2 d), 

 the sutural lines in the centro-sutural region are very distinctly marked. 



The mouth opening is large, being one half the diameter of the test at the equator, 

 the peristome is very unequally lobed, the ambulacral lobes being double the size of the 

 inter-ambulacral. 



The apical disc is absent, but as the opening is very large it indicates a great develop- 

 ment of the plates composing it, which, in the only four specimens I at present know, 

 are entirely absent. 



Two of the spines are fortunately preserved in our specimen (fig, 2 a,f) ; they are 

 cylindrical, with a prominent milled ring, and a small head (fig. 2/), the surface of the 

 stem is covered with longitudinal lines ; the stem is unfortunately broken, so that its length 

 is not known. 



Affinities and differences. — This urchin might easily be mistaken for a Hemicidaris, 

 like that genus it possesses two rows of semi-tubercles at the base of the ambulacra, 

 and two rows of primary tubercles in the inter- ambulacra; but the absence of crenulations 



