FROM THE GREAT OOLITE. 



243 



Description. — I have had much difficulty in making out the history of this species ; 

 and had it not been for the zeal, industry, and perseverance of my friend, the Rev. A. W. 

 Griesbach, in obtaining, for the sake of comparison, a fine series of specimens from Oundle, 

 Yardley, and Wollaston, Northamptonshire, I should have been unable to clear up the 

 doubts. For several years I regarded the large, inflated forms of this urchin, with 

 broad miliary zones, as granulated varieties of Acrosalenia Wiltonii ; the only perceptible 

 difference consisting in the size of the mouth opening, which is smaller in that species 

 (PI. XVI, fig. 3 b) than in Acrosalenia pustulata (PL XVI, fig. 2 b). The specimens which had 

 been collected in Gloucestershire, Oxfordshire and Northamptonshire, were, on an average, 

 about the size of the one figured in PL XVI, fig. 2, whilst Professor Forbes's type specimen 

 was only half the size of our urchins ; moreover, his diagnosis, always so correct in reference 

 to the object he described, "areolis disjunctis, area centrali angustissima, bigranulata, 

 granulis sparsis (diam. \ una, alt. \ una)/' did not agree with those I had hitherto 

 collected, for their areolas were confluent, and the area centralis was broad, with four rows 

 of granules. It occurred to me, therefore, that Professor Forbes's specimen, from the 

 Forest Marble of Malmesbury, was possibly an immature form of the species. I, therefore, 

 requested my friend, the Rev. A. W. Griesbach, to obtain specimens of the same age as 

 those described by Professor Forbes, which he fortunately discovered, and these small 

 urchins I found agreed very well with his diagnosis. 



The ambulacral areas are narrow and straight, they have two rows of small tubercles, 

 from eighteen to twenty in each row, disposed at some distance apart on the sides of the 

 area, those at the base (fig. 2 b) are a little larger than those on the sides (fig. 2 c) ; a 

 double series of granules takes the direction of the zigzag suture, and sends lateral branches 

 between their small areolas, so that on the upper part of the area these granules are nearly 

 as large as the small tubercles placed between them (fig. 2 c,d). 



The poriferous zones are narrow, the pairs of pores are oblique, and the septa form 

 moniliform elevations on the surface (fig. 2 d), which produce a bead-like line down the 

 centre of the zone between the pores ; and there are from eight to nine pairs of pores opposite 

 each large tubercular plate (fig. 2 d). 



The inter-ambulacral areas are nearly four times as wide as the ambulacral ; they have 

 eight plates in each column, of which the three equatorial pairs support large tubercles 



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