3i8 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. Fig. i2i,,—Hamites dttcnuatus. 



tuberculattis. 



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. 



