with a Synopsis of the Species included tha^ein. 95 



under the circumstances should have been attained, which will 

 bear the most severe criticism, and remain as it was left, a monu- 

 ment of the genius and industry of the authors. 



In our memoirs on the Cidaridse of the Oolites, we have 

 figured and described three species, Goniopygus perforatus, Pe- 

 dina Etheridgii and Pedina Bakeri ; the true generic position of 

 these forms seemed to us uncertain at the time our papers were 

 passing through the press, as they exhibited characters which 

 did not assimilate with either of the generic divisions of the 

 ' Catalogue raisonne/ Our materials did not then justify us in 

 proposing a separate genus for their reception ; the discovery, 

 however, of an interesting series of new congeneric forms has 

 now enabled us to rectify our determination, and propose the 

 genus Hemipedina for the group, to which we have added a 

 synopsis of the species included therein. 



Hemipedina, Wright, 1855. 



This new genus is composed of small, neat, and highly orna- 

 mented Urchins, much depressed on their upper surface, and 

 with a flat or slightly concave base. The ambulacral areas are 

 narrow and straight ; the pores in the poriferous zones are ar- 

 ranged in single pairs ; the interambulacral areas are in general 

 more than double the width of the ambulacral, with two, four, 

 or six rows of tubercles in general arranged abreast on the same 

 tubercular plate. The tubercles are perforated, and set on mam- 

 millary eminences with smooth uncrenulated summits ; one row 

 of tubercles in general only extends from the peristome to the 

 disc ; the other rows, when there are four and six rows in the 

 area, stop short at the equator, or between the equator and the 

 disc j the intertubercular space on the upper surface of the test is 

 therefore in general wide, and covered with a small miliary gra- 

 nulation. The apical disc is large ; the genital and ocular plates 

 are expanded and foliated. The mouth-opening is of moderate 

 dimensions, and the peristome is divided into ten nearly equal- 

 sized lobes. The spines are long, slender, and needle-shaped; 

 those that are known, equal at least the diameter of the test, and 

 their surface is sculptured with delicate longitudinal lines. 



Hemipedina is related to Diadema in having the pores ar- 

 ranged in the zones in single pairs and the tubercles perforated ; 

 but it is distinguished from Diadema by the absence of crenu- 

 lations on the summits of the mammillary eminences. It is , 

 related to Pedina in possessing perforated and uncrenulated 

 tubercles ; but it is distinguished from that genus in having the 

 pores in the zones in single pairs {Pedina having the pores in 

 triple oblique pairs like Echinus) j in having the elements of the 



