FROM THE CORAL RAG. J 61 



Locality and Stratigraphical position. — It appears to have been collected from the 

 Corn brash of Northamptonshire ; but as the history of the specimen is unknown, this 

 is only conjecture. It forms part of the late Miss Baker's (Northampton) Collection in the 

 British Museum. 



D. Species from the Coral Rag. 



Hemipedina Marchamensis. Wright. PI. XI, fig. 1 a, b. 



Hemipedina Marchamensis. Wright, Annals and Magazine of Natural History, 2nd series, 



vol. xvi, p. 197. 

 — — Woodward, Memoirs of the Geological Survey, Decade v, 



" Notes on Echinopsis." 



Test large, circular, and depressed ; ambulacral areas narrow, with two rows of 

 marginal tubercles, nearly as large as those of the inter-ambulacral ; extending 

 regularly, without interruption, from the peristome to the apical disc, and separated by a 

 zig-zag line of small granules, the areas retaining a nearly uniform width throughout ; 

 poriferous zones narrow, forming a slightly waved line, every three pairs of pores being set 

 obliquely in the line of the zones ; inter-ambulacral areas four times the width of the 

 ambulacral, with eight rows of tubercles at the equator, each tubercular plate supporting 

 four nearly equal sized tubercles abreast ; mouth opening large, peristome unequally 

 decagonal. 



Dimensions. — Transverse diameter, two inches and nine tenths; height, one inch 

 and three tenths. 



Description. — In the present state of our knowledge, it would be premature to propose 

 subgenera of the remarkable group of urchins now under consideration, but it is evident 

 that we have at least two sections of the genus Hemipedina, among its forms now known. 

 1st, those with two rows of primary tubercles, a wide miliary zone, and sometimes 

 rows of secondary tubercles in the inter-ambulacral areas. 2d, those with four, six, 

 eight, or even ten rows of nearly equal sized tubercles abreast, in the same region of the 

 test ; Hemipedina Woodwardi, and Hemipedina tuberculosa, are types of the first section ; 

 Hemipedina Marchamensis, and Hemipedina Bouchardii, are types of the second. At 

 first sight, it seems difficult to believe that the two urchins, figured in PI. XI, fig. 1 and 2, 

 belong to the same genus, but a careful analysis of their structure does not afford any 

 permanent characters for generic separation. 



A parallel case is afforded by the genus Pseudodiadema, where one section has two 



