94 
the length ; the lobe is distinct, but not sharply defined ; 
the surface is smooth, and nearly free from laminae. 
Casts in Silex, supplying the place of the shell itself, 
are very abundantly collected in masses, in loose green 
sand stained by iron, occurring near Warminster, and 
generally unassociated with other fossils. I am obliged 
to Miss Benett for the specimens figured. 
