FROM THE WHITE CHALK. 133 



contracted, and provided with two rows of granules closely set together, unequal in size 

 some of them being mammillated at the angles of the plates. 



The poriferous zones are narrow and undulated at the base and ambitus, and wide and 

 straight at the upper surface ; the pores are small, and unigeminal from the peristome to 

 the ambitus (fig. 1 e), and distinctly bigeminal on more than the upper third of the 

 zones (fig. 1 d) ; near the peristome the pairs are doubled (fig. 1 g). 



The inter-ambulacral areas are about one third wider than the ambulacral, and furnished 

 with primary and secondary tubercles ; the primary series consists of two rows, twelve in 

 each, occupying the centre of the plates ; the tubercles are smaller on the upper than on 

 the lower third of the area ; the secondary tubercles are large and unequal in size ; they 

 form a series between the poriferous zones and primary tubercles, some of which they 

 resemble in magnitude, especially those extending from the ambitus to the coronal plates ; 

 between the ambitus and peristome they are much smaller, and in many specimens are 

 not more developed than large granules on mammillated eminences. 



The miliary zone is very wide, naked, and depressed at the upper surface, 

 becoming gradually narrower at the ambitus, and much contracted at the base; the granules, 

 unequal in size, are fine, abundant, set closely together, and arranged in semicircles 

 around the primary and secondary tubercles ; some of the granules, much larger than the 

 others, are set on mammillated eminences, and may easily be mistaken for the small 

 secondary tubercles placed near them ; this mnigling together of small tubercles and large 

 mammillated granules imparts a highly ornamented character to the infra- marginal region 

 of this species. 



Discal opening large, pentagonal, and subangular, the elements wanting in all the 

 specimens hitherto found. Mouth-opening small, circular, the peristome feebly incised, 

 the border reflected, the entailles having the border elevated and opening upwards. , 



The spines exhibit some remarkable variations from the typical form of structure. 

 Some of the most curious of these I have figured in Pis. XXIV and XXVI. The 

 typical spines, as seen in those in situ in the specimen belonging the British Museum 

 (Pi. XXIV, fig. a), are elongated, subcylindrical, and sometimes aciculate at the summit 

 (fig. 1 d, and fig. 3 ci), or spatuliforra with carinae, as PI. XXIV, fig. 1 c, or spoon- 

 shaped, as PI. XXVI, figs. 1 a, b, the lower part or coUerette being long and distinct, 

 and covered with fine longitudinal lines (PI. XXIV, fig. 3 b), much stronger than those 

 observed on the stem ; the head is well developed, the milled ring very prominent, flat, 

 and deeply striated, and the rim of the acetabulum finely crenulated (PI. XXVI, figs. 1 

 a, c, figs. 2, 4, 6), with other varieties of spines figured in this plate. 



The test of this Urchin presents many variations of form from the typical shape seen 

 in the specimen figured in PI. XXIII, fig. 1 a, h, and considered to be its normal form. 

 In PI. XXIV, fig. 7, I have figured a remarkable monstrosity of this species from 

 the British Museum Collection ; it is inversely conical, like Pseudodiadema tumidmn ; 

 measures three quarters of an inch in height, and is one inch and one third in diameter 



