156 HEMIPEDINA. 



throughout, and there are three pairs of pores opposite each of the inter-ambulacral plates 

 (fig. 5 d). 



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

 there are six rows of tubercles (fig. 5 d), nearly of the same size, but only the central row 

 of each column extends from the peristome to the disc, at the base this row is large and 

 very conspicuous (fig. 5 c), at the equator the tubercles are a little larger than 

 the lateral rows, and from the equator upwards (fig. 5 b), they alone occupy the upper 

 surface of the shell, of the two lateral rows, that on the zonal side is the shortest ; 

 there are from sixteen to eighteen tubercles in each of the central rows, and from 

 eight to ten in the lateral rows ; the areolas are surrounded by small granules, which 

 form a delicate network on the surface of the plates, (fig. 5 d). 



The apical disc is absent in all the specimens I have seen, it has an oblong form, and 

 extends further into the single inter-ambulacrum than the other areas (fig. 5 c). 



The base is slightly concave (fig. 5 c), and the tubercles are much larger in this region 

 than on the upper surface (fig. 5 b) ; the mouth opening is large, and the peristome is 

 divided into nearly equal sized lobes. 



Affinities and differences. — The pentagonal form of this species, with its elongated 

 opening for the apical disc, reminds me of some young Piasters, which occur with it in 

 the same rock ; but the size and arrangement of the tubercles, its depressed form, and 

 small discal opening, serve to distinguish it from them ; how far a greater number 

 of specimens, especially if the apical disc were preserved, would lead me to modify my 

 opinion as to its generic position, it is impossible to say ; I have, therefore, provisionally 

 placed this beautiful little urchin with the Hemipedinas, the characters just enumerated 

 being sufficient to show in what it differs from its congeners. 



Locality and Stratigraphical position. — I have collected this urchin from the Pea grit, 

 Inferior Oolite, of Crickley Hill, associated with the preceding species from the same bed, 

 where it is rare. I dedicate this species to Mr. C. R. Bone, whose beautiful figures of 

 Echinoderms, have given such a lasting value to the plates which accompany this 

 Monograph. 



C Species from the Great Oolite and Cornbrash. 



Hemipedina Davidsoni, Wright. PI. XII, fig. 6 a, b, c, d. 



Hemipedina Davidsoni. Wright, Annals and Magazine of Natural History, 2d series, 



vol. xvi, p. 99. 

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

 on Echinopsis." 



