THE 'DERIVED' FOSSILS. 43 



The absence of Ceritkium Portlandicum, tlie " Portland Screw," 

 is noteworthy, but may be accounted for by the fragile nature of 

 the casts of these shells. 



KlMMERIDGE CLAY FOSSILS. 



Kimmeridge clay fossils are amongst the most abundant of our 

 derived shells. The following have been recognised : 



Belemnites explanatus,~P\u\\..; Ammojiitesbiplex,Sby.', Ammon- 

 ites Kcenigi, Sby. ; Pleurotomaria reticulata, Sby. ; Trochus, sp. 

 and other Gasteropoda (Alaria, Chemnitzia, &c.); Gervillia avicu- 

 loides, Sby.; ? Lima Iceviuscula, Sby.; Cardium striatulum, Sby.; 

 Myoconcha Scemanni ; Astarte Hartwelliensis, Sby. ; Myacites re- 

 curva, Phill. ? ; Myacites, sp.; Serpula tricarinata (affixed to the 

 Belemnites). 



These are all (except the Belemnites) internal casts in a phos- 

 phatized condition. 



Besides these invertebrate species the Kimmeridge clay seems 

 to have furnished, by its destruction, some of the vertebrate 

 remains now found in our Neocomians. I have no doubt that 

 many of the Saurian remains are of Kimmeridge clay origin, 

 amongst which some of the Ichthyosaurs, Plesiosaurs, and Pliosaurs 

 are the most certain. Our 'Potton sand' specimens compare 

 well with Jurassic species, and we have, besides, other positive 

 evidence of Kimmeridge clay remanie material amongst the Mollus- 

 can remains. The Upware bed has furnished us with the teeth of 

 Pliosaurus brachydeirus, both the three-sided and rounded forms, 

 precisely like those of the Ely clay-pit (Kimmeridge clay); the 

 Dakosaur teeth are likewise identical, and some of the Plesiosau- 

 rian, Ichthyosaurian, and Crocodilian teeth have probably the 

 same origin. 



It will however be remembered that very many of the reptiles 

 of our Ironsand and Phosphatic beds were living in the Neocomian 

 period, and are true natives of the deposit. 



Amongst the Fishes are some extremely well-preserved teeth 

 and jaws belonging to Sphcerodus, Gyrodus, Pycnodus, Strophodus, 

 and the spines of Aster acanthus, which are absolutely undistinguish- 

 able from the Jurassic species. Many of these are, in my opinion, 

 'derived' fossils of Jurassic age, but I fail fully to separate out 

 these from others which certainly swam in the Neocomian sea. 



