i6 Enlaro-ement of a specimen without the superficial ornament ; showino- 

 stronof concentric ia:rowth lines 



1 7 A normal example with the sculpture retained on the concentric 



corrugations, x 2 



18 A rici-ht valve in which the concentric structure is suppressed and the 



superficial ornament relatively pronounced, x 2 



19 Enlargement of the posterior portion of a flattened left valve showing 



the coarse and fine concentric lines, x 3 

 Grande Grcvc limestone; division 2. At various points along the 

 Forillon 



301 



