FROM THE GREAT OOLITE. 157 



Test circular, much depressed, ambulacral areas with two marginal rows of tubercles 

 very regularly arranged throughout the area ; inter-ambulacral areas wide, with two 

 central rows of primary tubercles, and four lateral rows of secondary tubercles ; poriferous 

 zones narrow, and straight ; under surface of the test crowded with tubercles, upper surface 

 deficient in tubercles, base concave, mouth opening small, peristome unequally deca- 

 gonal. 



Dimensions. — Height, seven twentieths of an inch ; transverse diameter, one inch. 



Description. — I found this urchin about twelve years ago in the sandy beds of the 

 Great Oolite, at Minchinhampton ; and although I have searched diligently since in the 

 same locality for other specimens, I have failed. The test is not in very good preservation, 

 but it is still sufficiently so to enable me to describe its characters ; the shell is thin, cir- 

 cular, and much depressed ; the ambulacral areas are straight and narrow (fig. 6 a), being 

 less than one third the width of the inter-ambulacral, on their margins are two rows of 

 tubercles (fig. 6 d) in each row, which are very uniform in size throughout, corresponding 

 with a greater uniformity in the width of the area than in some other species ; the pori- 

 ferous zones are narrow ; the pores unigeminal throughout (fig. 6 a), except at the base, 

 where three additional pairs are crowded in ; there are four pairs of pores opposite each 

 inter-ambulacral plate. 



The inter-ambulacral areas are three times the width of the ambulacral ; each of the 

 elongated pentagonal plates composing the two columns has one central primary 

 tubercle (fig. 6 d) ; these tubercles form an uninterrupted row, which extends from the 

 peristome to the disc ; besides the central row, the plates between the peristome and 

 equator have two lateral rows of smaller secondary tubercles (fig. 6 d), disposed on 

 each side of the primary one, so that at the equator the inter-ambulacra have six rows 

 of tubercles abreast ; above that line, however, the secondary series disappear, and on 

 the upper surface of the test the two primary rows alone exist (fig. 6 b) ; the primary 

 tubercles are very uniform in size throughout, but the secondaries vary much in magnitude 

 (fig. 6 d). 



The base is flat, the mouth opening is small, being about one third the diameter of the 

 shell, the peristome is decagonal and nearly equal-lobed (fig. 6 c). 



Affinities and differences. — This species resembles Hemipedina tetragramma, but the 

 ambulacral tubercles are larger and less numerous ; the poriferous zones are narrower, and 

 the pores more strictly unigeminal ; the inter-ambulacral areas are wider, and less 

 ornamented with granules and tubercles; the secondary tubercles between the pori- 

 ferous zones and the primary row (fig. 6 d), is absent in Hemipedina tetragramma. PL X, 

 fig. 3 d, the aperture for the apical disc is likewise much larger in Hemipedina Davidsoni. 



