158 HEMIPEDINA. 



Locality and Stratigraphical position. — I collected this urchin from the sandy beds of 

 the Great Oolite, at Minchinhampton Common, where it is extremely rare ; the specimen 

 figured is the only one I know. I dedicate this species to my esteemed friend, Thomas 

 Davidson, Esq., author of the ' Monographs on the Fossil Brachiopoda of Great Britain.' 



Hemipedina Woodwarm, Wright. PI. XII, fig. 7 a, b, c, d. 



Hemipedina Woodwakdi. Wright, Annals and Magazine of Natural History, 2nd series, 



vol. xvi, p. 99. 

 — — Woodward, Memoirs of the Geological Survey, Decade v, " Notes 



on Echinopsis." 



Test small, circular, and much depressed ; ambulacral areas narrow, with two rows of 

 small tubercles which extend from the base to the equator, and diminish to small granules 

 in the upper part of the areas ; inter-ambulacral areas with two rows of primary tubercles, 

 eight in each row, and two rows of secondary tubercles, three to four in each row, which 

 scarcely reach the equator ; the miliary zone is wide on the upper surface and filled with 

 numerous close-set granulations ; apical disc large, genital plates much developed ; mouth 

 opening small, peristome decagonal and nearly equal lobed. 



Dimensions. — Small specimen, height seven twentieths of an inch ; transverse diameter 

 thirteen twentieths of an inch. 



Description. — This urchin belongs to the type of Hemipedinas, which have two rows 

 of primary tubercles, with a wide granulated miliary zone between them on the upper 

 surface of the inter-ambulacral areas ; although undescribed, this species has been long 

 known, as there is a fragment of it, collected by Dr. William Smith, from the Cornbrash, 

 which forms part of his collection now in the British Museum. 



The ambulacral areas are straight and narrow (fig. 7 a), on their lower half, or from 

 the peristome to above the equator (fig. 7 c), there are from nine to eleven pairs of 

 tubercles according to the size of the specimen ; these tubercles are perforated, and raised 

 on bosses with semicircles of minute granules around them (fig. 7 d) ; from about the 

 equator to the apical disc the tubercles degenerate into granules (fig. 7 b), so that the 

 lower half of the area possesses tubercles, whilst the upper half is occupied by granules ; 

 the poriferous zones are narrow, and straight ; the pores, which are strictly unigeminal, 

 are small (fig. Id), and there are five pairs of pores opposite each of the inter-ambulacral 

 plates ; the septa have slightly raised eminences on the surface. 



The inter-ambulacral areas are four times the width of the ambulacral, they have two 

 entire rows of primary tubercles, and two short rows of secondary tubercles ; each plate, 

 between the peristome and the equator, has one large tubercle on its zonal side, and one 



