f 



BRITISH FOSSILS. 3 



sinuous elevated space ornamented with a double row of granules 

 (secondary tubercles) running down the centre of each interambulacral 

 area, and a similar single row separating the ambulacral area on each 

 side from the poriferous avenues. The ambulacral areas bear very- 

 small spiniferous tubercles compared to those on the interambulacrals. 

 On their upper and middle portion the tubercles are very small, but 

 just below the largest interambulacral plate increase suddenly, then 

 gradually diminish as they approach the mouth. There are about seven 

 in a row (or fourteen in each ambulacral area) of the larger tubercles, 

 of which, however, those nearest the mouth are as small as the 

 ambulacral tubercles of the upper and middle portions of the area. 

 Like the large primary bosses of the interambulacral plates, those of 

 the ambulacrals also are crenulated at their summits below the per- 

 forated tubercles which crown them. The lower and larger ones are 

 separated from each other by a few occasional granules, the upper ones 

 are more distinctly bordered by minute granules. As there is only one 

 spiniferous tubercle to each ambulacral plate, the effect of the ambu- 

 lacral area is that of a widening space studded by closely-set tubercles, 

 ranged in two rows, and suddenly increasing in size at the lower 

 margin of the sides of the test. The avenues of pores are slightly 

 sinuous ; the pairs of pores are ranged in single file as far as just below 

 the sudden increase in the size of the ambulacral tubercles, when they 

 become irregular and arranged in oblique rows of three pair in each. 

 There are five or six of such rows extending to the mouth and causing 

 the avenues to widen out considerably in the buccal region. There are 

 nine or ten pair of pores opposite each of the larger plates. 



The mouth is very wide, occupying three-fifths of the under-surface ; 

 its margin is deeply notched and reflexed at the junction between each 

 interambulacral series of plates and the poriferous avenues. There 

 results a division of the mouth-margin into ten segments, five (those 

 opposite the ambulacral spaces) wider than the other five. 



The genital disk occupies a fourth of the diameter of the test. It is 

 pentagonal, composed of five genital and five ocular plates. Three of 

 the former are larger than the other two ; one of the three is the 

 madreporiform plate. There is not so great a disparity in the dimen- 

 sions of the ocular plates. The effect of this arrangement is to cause 

 an eccentricity of the anal perforation. Both genital and ocular plates 

 are irregularly studded with numerous small granules. The genital 

 pores are placed in the lower portions of their plates. The eye-holes 

 are very minute, marginal, and opposite the truncated ends of the 

 ambulacral spaces. 



The primary spines are long, tapering, and nearly cylindrical ; the 

 larger ones grow to a length more than twice that of the diameter of 



