FROM THE MARINE PURBECK BEDS. 101 



them is seen to be very great. The size and prominence of the inter-ambulacral tubercles 

 in Ilemicidaris stramonium, the magnitude and spaced-out arrangement of the semi- 

 tubercles at the base of the ambulacra, and the rudimentary condition of the marginal 

 granules on the upper half of these areas, form a group of characters by which this species 

 is readily distinguished from its congeners. It belongs, moreover, to beds newer than 

 the Kimmeridge Clay. 



This species must, therefore, be omitted from our list of British Ilemicidaris. 



