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On a New Genus of Fossil Cidaridae, with a Synopsis of the 

 Species included therein. By THOMAS WRIGHT, M.D., F.R.S.E. 



NOTWITHSTANDING the many new generic sections introduced 

 into the classification of Echinoderms, by MM. Agassiz and 

 Desor, and the important light thrown by these savans on our 

 knowledge of the numerous species of this class contained in 

 European collections, still the progress of discovery renders it 

 imperative on palaeontologists to modify from time to time 

 many of the opinions put forward by these authors in their 

 ' Catalogue raisonneV When the amount of real work done by 

 them is taken into account, in a field which was then compara- 

 tively unknown, the wonder is, not that mistakes or oversights 

 should have been committed, but that so much good work 

 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 Cidaridae of the Oolites, we have 

 figured and described three species, Goniapygus 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 raisonneV 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 arc perforated, and set on mam- 

 nullary 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 



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