150 HEMIPEDINA. 



Test circular, depressed ; ambulacral areas narrow, with one row of small tubercles, 

 disposed in a slightly zigzag line, down the centre of the areas ; inter-ambulacral areas 

 broad, with two rows of primary tubercles in the centre of the plates, raised on prominent 

 bosses ; margins of the areolas surrounded by circles of small granules ; no secondary 

 tubercles. 



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

 of an inch. 



Description. — This rare urchin has the test circular and depressed (fig. 1 b, c); the 

 ambulacral areas are narrow, about one third the width of the inter-ambulacral ; the usual 

 double row of tubercles in this region is reduced to one row, the tubercles of which are 

 disposed alternately on the right and left sides of the areas, thereby forming a single zigzag 

 line down the centre thereof (fig. \ h, c, d); the tubercles at the equator, and on the 

 upper surface, are small (fig. 1 b), but there are two or three of a larger size at the base 

 of the areas (fig. 1 ^ ) ; a few granules form imperfect scrobicular crescents round the 

 narrow areolas (fig. \ d). The inter-ambulacral areas are nearly three times the width of 

 the ambulacral (fig. 1 b) ; they are adorned with five pairs of primary tubercles, nearly 

 of a uniform size throughout (fig. \h,c,d), raised on prominent bosses, the summits 

 of "which are smooth, ring-like, and without crenulations (fig. \ d, e) ; circles of small 

 granules bound the areolar spaces ; there are no secondary tubercles, nor any other 

 sculpture upon the inter-tubercular surface of the plates, so that down the centre of the 

 areas there is a smooth valley between the primary tubercles (fig. 1 d, c, b). The apical 

 disc is well preserved (fig. 1 b,f); the ovarial plates are of moderate size, and have an 

 irregular, heptagonal form ; they are covered with a few granules, scattered irregularly over 

 their surface ; the ocular plates are rhomboidal, and have large eye-holes perforated near 

 the centre (fig. 1/). 



The base is flat (fig. 1 c), and the mouth opening small (fig 1 a) ; the peristome is 

 divided into ten lobes by shallow notches. 



Affinities and differences. — This species differs so much from all its congeners, that it 

 cannot be mistaken for either of them. Its diagnostic characters consist in the size and 

 number of the primary tubercles (fig. 1 b), the absence of secondary tubercles, the naked 

 valley in the centre of the miliary zone (fig. 1 c, d), the narrowness of the ambulacra, with 

 the single row of tubercles therein (fig. \ d). 



Locality and Stratigrapldcal position. — 1 have collected only one specimen of this 

 singular form in the Pea Grit at Crickley Hill, and have seen fragments only on the 

 surface of other fossils, as one or two plates suffice for the determination of this species. 

 I dedicate this fine urchin to ray friend T. Barwick L. Baker, Esq., of Hardwicke Court, 

 Gloucestershire, the President of the Cotteswold Naturalists' Club. 



