3i 8 OUR COMMON BRITISH FOSSILS. 



purely argillaceous or clayey origin, the fossil Ammo- 

 nites still retain their pearly, iridescent nacre, or 

 "mother-of-pearl" coating. This is the case with 

 them in the Oxford Clay, and also in the Gault a 

 subdivision of the Cretaceous or Chalk formation. 

 Indeed, in the latter the fossils are perhaps more 



Fig. 323. Fragment of T. 

 t^lbercnlatus. 



Fig. 324. Hamites attenuates. 



beautifully preserved in this respect than in any other 

 formation, although I have laid open Strophomena 

 and other fossil Brachiopods in the Wenlock Shales 

 (another clayey deposit), which showed distinct 

 nacreous lustre at first, but immediately vanished on 

 exposure to the air. 



