106 PSEUDODIADEMA 



ambitus are very largely developed, and occupy the greatest part of the area, having large 

 areolas, prominent bosses, sharply crenulated summits, with projecting, deeply perforated 

 mammillons (PL XXI, figS d); on the upper surface they rapidly diminish in size ; four of 

 them have small areolas and the others become mere granules. In the infra-marginal region 

 they gradually diminish as they approach the peristome, where two short rows of secondary 

 tubercles are regularly arranged. The primary tubercles are separated by two rows of very 

 fine granules, which gradually increase in number, and fill the entire upper portion of the 

 area (fig. 3 d). 



The poriferous zones are narrow and straight at the base, undulated on the sides, and 

 straight again on the upper surface; the pores are simple, round, largely open, and 

 arranged in a single file throughout (fig. 3 c and d). 



The inter-ambulacral areas are relatively small from the excessive width of the ambu- 

 lacra ; they have two rows of primary tubercles, of which three pairs at the ambitus, like 

 those in the ambulacral areas, attain a great development ; above they become suddenly 

 smaller, and diminish to mere granules ; and on the infra-marginal region they become 

 gradually smaller as they approach the peristome. The secondary tubercles are very abun- 

 dant, and limited to this region of the test ; at the base of each interambulacra there are 

 six short rows, a long and a short row between the zones and the tubercles on each side, 

 and two short rows between the tubercles themselves. This great profusion of small 

 secondary tubercles and diminished size of those in the primary series impart to the base 

 of the test of this Urchin a remarkable ornamental appearance, which resembles, on a larger 

 scale, the structure of the base in P. Mhodani (fig. 3 h). 



The miliary zone is largely developed ; the two primary rows of tubercles at the 

 ambitus have six rows of granules forming a band between them ; in the upper part the 

 granulation increases, and fills the whole zone, except the part occupied by the areolas of 

 the small rudimentary tubercles, forming circles around them, and filling the whole space 

 Avitli a fine nearly uniform granulation. 



The base is very concave and crowded with small tubercles ; the mouth-opening is 

 small, and lies at the bottom of a very deep depression ; the peristome is narrow, and 

 marked by feeble indentations. 



Jffinities and Differences.- — This remarkable species resembles P. Rhodanim having its 

 base crowded with numerous small tubercles, and in having large tubercles at the ambitus, 

 and rudimentary ones on the upper surface. Its form, however, is always inflated ; the 

 poriferous zones are straight at the base and upper surface, and undulated at the ambitus. 

 The secondary tubercles are larger and more numerous, and distinguished with difficulty 

 from those of the primary rows of the under surface. 



Loccdity and Straiigra^phical Position. — The only specimen I have seen was obtained 

 from the Grey Chalk near Folkestone by the Rev. T. Wiltshire, to whose cabinet it 

 belongs. — Foreign Distribution. — M. Cotteau records Vimoutiers (Orne), in the Etage 

 Cenomanien, where it is very rare. 



