122 PSEUDODIADEMA 



straight, with two rows of small primary tubercles (fig. 2 c), about thirteen in each row ; 

 their bosses are small, and set closely together; a zig-zag central line of granules descends 

 between the rows, and a few scattered granules separate the areolas above and below from 

 each other. 



The poriferous zones are straight and narrow, the pores are unigeminal and contiguous, 

 and the pairs of pores are obliquely inclined (fig. 2 c) ; three pairs of holes are opposite 

 one ambulacral plate, and five pairs are opposite one inter-ambulacral plate (fig. 2 c) ; near 

 the peristome the pores fall into triple oblique pairs. 



The inter-ambulacral areas are rather more than twice and a half as wide as the 

 ambulacral ; they have two rows of primary tubercles, with thirteen in each row ; they 

 are considerably larger than those of the ambulacral areas ; their bosses have a broad base, 

 but their summits are not deeply crenulated (fig. 2 c) ; the spinigerous tubercles are small, 

 flat, and minutely perforated ; the central miliary zone is wide, but sparingly granulated 

 in the middle, and almost naked above ; on each plate (fig. 2 c) two rows of granules form 

 a lateral zone, separating the tubercles from the poriferous avenues, and two rows of 

 granules form one half of the central zone ; in the upper part of the area the areolas are 

 separated by transverse rows of granules, which, with the lateral rows, form three fourths 

 of a scrobicular circle. The weathered condition of the specimen renders a more minute 

 description of the test impossible. 



The mouth opening measures eleven twentieths of an inch, that of the diameter of 

 the test being one inch and four tenths ; the peristome is deeply notched, and unequally 

 lobed, the inter-ambulacral being one third less than the ambulacral lobes. 



The disc opening is four tenths of an inch in diameter, but the plates are absent. 



Affinities and differences. — This species very much resembles the larger specimens of 

 Pseudodiadema depression, but the ambulacral tubercles appear to be proportionately 

 smaller, and the ambulacral areas narrower than in that species ; the bosses, likewise, are 

 not so prominent, and the spinigerous tubercles are smaller ; but from the weathered 

 condition of the specimen, the only one known, it is impossible to make a more accurate 

 comparison between these two nearly allied forms. 



Locality and Stratigraphical position. — This urchin was collected from the Cornbrash 

 at Caistor, Northamptonshire, by the late Miss Baker, and belongs to the British Museum 

 Collection. It has been kindly communicated for this work by Mr. S. P. Woodward, 

 who first described it in his excellent Notes on British Fossil Diademas, inserted in the 

 5th Decade of the ' Memoirs of the Geological Survey.' 



