166 HEMIPEDINA. 



from the summits of the bosses, the presence of secondary tubercles in the inter-ambulacra, 

 and the great size of the discal opening, are characters which serve to distinguish it from 

 Hemicidaris. 



It forms the best type of that section of the genus, which has two rows of tubercles 

 in the inter-ambulacra, with a wide miliary zone; the contrast between the two 

 sections of the group is strikingly exemplified by comparing figures 1 and 2 of Plate XL 

 where Hemipedina Marchamensis, and Hemipedina tuberculosa are drawn side by side. 



In the structure of the ambulacral areas, with tubercles below, and granules above, and 

 with two rows of tubercles in the inter-ambulacra, this species resembles Hemipedina 

 Woodwardi, but the tubercles in Hemipedina tuberculosa are much larger, the granules 

 are more developed, and it is a higher and more inflated form than the Cornbrash species. 

 Our urchin very much resembles the mould in plaster named Hemicidaris depressa, 

 Agassiz, "X 55, R 44. Cat. Syst. p. 8. Espece plate, subconique, a, ambulacres non 

 flexueux" from the Forest Marble, of Ranvilie,*" but it is impossible to make a critical 

 comparison of that form with Hemipedina tuberculosa, without a typical specimen ; at 

 present I know of no other form for which this beautiful urchin could be mistaken. 



Locality and Stratigraphical position. — This species has been found only in the Coral 

 Rag of Wiltshire, by Mr. William Buy ; the fine specimen I have figured, was discovered 

 by him near Lyneham ; it is a very rare form, as that able and accurate collector has only 

 found three or four specimens of the species. 



E. Species from the Kimmeridye Clay. 

 Hemipedina Morrisii, PI. XII, fig. 2 a, b, c, d, e. 



Hemipedina Morrisii. Wright, Annals and Magazine of Natural History, 2d series, 



vol. xvi, p. 198. 

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



on Echinopsis." 



Form and size unknown ; test small ; ambulacral areas with two rows of prominent 

 marginal tubercles ; poriferous zones slightly waved ; inter-ambulacral areas with four rows 

 of prominent tubercles at the equator, nearly on the same line, surrounded by incomplete 

 circlets of granules. 



Description. — The only portion of this urchin I have seen, is that now figured ; 



* Annales des Sciences Naturelles, 3 im " serie, tome vi, p. 338, Agassiz and Desor. Catalogue raisonne 

 des Echinides. 



