HEMIPEDINA. 143 



Genus—UEMlVEDmA, Wright. 1855. 



This rare genus is composed of neat and highly ornamented urchins, in general much 

 depressed on their upper surface, and with a flat or slightly concave base. The arabu- 

 lacral areas are narrow and straight ; the pores in the poriferous zones are arranged in 

 single pairs ; the inter-ambulacral areas are, in general, more than double the width of the 

 ambulacral, with two, four, six, or eight rows of tubercles, arranged in general abreast on 

 the same tubercular plate. 



The tubercles are perforated, and set on bosses with smooth, uncrenulated summits; 

 one row of tubercles 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 ; the upper part of the miliary zone is therefore in general wide, and covered with a 

 fine granulation. 



The apical disc is large ; the genital plates are expanded and foliated ; the eye holes 

 are perforated in the centre of the ocular plates, which are large. 



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 m 

 length the diameter of the test, and their surface is sculptured with delicate, longitudinal 

 lines. 



Hemipedina is related to Pseudodiadema in having the pores unigeminal, and the 

 tubercles perforated ; but is distinguished from Pseudodiadema by the absence of crenu- 

 lations from the summits of the bosses. 



It is related to Pedina in possessing perforated and uncrenulated tubercles ; but is 

 distinguished from that genus in having the pores unigeminal {Pedina having the pores 

 trigeminal like Echinus) ; the elements of the apical disc are likewise more largely 

 developed. 



Hemipedina is related to JEchinopsis in possessing uncrenulated and perforated 

 tubercles, with unigeminal pores ; but is distinguished from Echinojms by the nar- 

 rowness of the ambulacral areas, the general depressed form of the test, the shape of the 

 mouth opening, and the deep decagonal lobes of the peristome (that of Echinopsis being 

 almost deprived of incisions), together with the greater size and development of the 

 elements of the apical disc. 



Hemipedina, as far as we at present know, is composed of Oolitic species which com- 

 menced in the Lower Lias, and extended into the upper division of the Oolites, each stage 

 possessing its own specific forms. 



