FROM THE WHITE CHALK. 55 



depressed along the line of all the sutures ; apical disc wide ; ovarial plates thick, large ; 

 ocular plates heart-shaped j spines long, cylindrical, fusiform, enlarged at the inferior third, 

 and tapering gently to the summit ; surface covered with regular, longitudinal rows of 

 spiny, projecting granules, the intervening space being finely shagreened. 



Dimensions. — Height, one inch and four tenths ; transverse diameter, one inch and 

 three tenths. 



Description. — A very good figure of a fragment of the test and spines of this urchin 

 was given in Parkinson's ' Organic Remains/ 1 pi. iv, fig. 2, and a fair drawing of the 

 upper surface of the shell in pi. i, fig. 11; both were, unfortunately, unaccompanied by 

 any description. 



This beautiful species attains a considerable size ; the test is circular, and nearly 

 equally depressed at both poles (PI. VI, fig. 1, c) ; the ambulacral areas are very narrow 

 and flexuous, with six rows of granules at the equator, diminishing to four rows at both 

 poles ; the external rows are the largest and most persistent, and the internal rows are 

 found only in the wide portion of the area ; in large shells there are many smaller granules, 

 dispersed among the regular rows (PI. VI, fig. 1, d) ; the poriferous zones are very narrow 

 and depressed, following the flexures of the areas ; the pores are small, round, closely 

 approximated, and disposed obliquely, having a minute granule on the septum, and there 

 are twenty-four to twenty-six pores opposite one of the larger plates (fig. 1, d). 



The inter-ambulacral areas are very wide, and there are from four to five large plates in a 

 column ; in consequence of the prominence of the scrobicular circle, each plate is thickened 

 at the middle, and the lines of sutures between the plates are all very much defined, in 

 consequence of the plate sloping away from the scrobicular circle to the suture ; the areolas 

 are wide and circular, and surrounded by a thick, prominent border (fig. 1, d), encircled by a 

 series of seventeen granules, each raised on a distinct, shield-like, mammillated plate ; the 

 boss is not prominent, its summit is feebly crenulated in young shells, and smooth in old 

 ones (PI. VI, fig. 1) ; the tubercle is moderately large and perforated; the plates near the 

 peristome (PI. VI, fig. 1, b) are disproportionately small compared with those of the 

 equator (fig. 1, c, d) ; and the last plate of each column in old shells (fig. 1, a, e,f) has 

 an elongated form, with a rudimentary tubercle, and curious elongated furrow, representing 

 an obsolete areola. Mr. Bone has given most accurate drawings of this remarkable plate 

 from the fine specimen in the British Museum ; the singular star-like structure which 

 these plates form on the upper surface of the test, surrounding the large apical disc, forms 

 a remarkable character of this species, fig. 1, a. 



The miliary zone is narrow, zig-zag, depressed in the middle, and provided with fine,close- 

 set, nearly equal-sized granules, very much smaller than those surrounding the areolas. 



1 'Organic Remaius,' vol. iii, pi. iv, fig. 2. 



