44 THE 'derived' fossils. 



Besides these there are Hybodus (dorsal spines, teeth, and 

 Sphenonchus), Otodus, Acrodus and Chimceroids, all of which would 

 be inscribed by Mr Walker amongst the 'derived' fossils. 



Coral Rag. 



At Upware the Coprolite bed rests, in places, upon the Coral 

 Rag, which was broken up by the waves so as to form a rubbly 

 zone of fragments along the line of junction (see the section, p. 4) 

 so that the origin of the following species found in the Upware 

 Neocomians is not far to seek. 



Chemnitzia Heddingtonensis, Sby. ; Cerithium muricatum Sby.; 

 Gryphea dilatata, var.; Ostrea gregaria; Exogyra; Pecten vimi- 

 neus; Lithodomus inclusus; Opis corallina; Unicardium; Hemi- 

 cidaris intermedia (flattened form); Pseudodiadema hemispheri- 

 cum ; Cidaris florigemma ; Echinobrissus scutatus ; Holectypus 

 depressus; Glypticus hieroglyphicus; ? Apiocrinus or Millericrinus. 



Now it is a remarkable fact that these fossils have not become 

 phosphatised but retain their original condition as seen in the 

 Coral Rag pits near by. 



But between Upware and Brickhill the Coral Rag is not 

 developed as such, being represented by some coeval argillaceous 

 clay deposits (the Ampthill clay of Prof. Seeley), and it is not 

 unlikely that some of the species such as Gervillia aviculoides and 

 Lima Iceviuscida, which we have placed in other lists, may have 

 been derived from rocks of this age. 



The Derived Fossils from the Oxford Clay. 



This formation must have formed a considerable part of the 

 shore line of the old Neocomian sea, for in many places the Iron- 

 sand and Nodule beds rest upon it. The Ammonites of this age 

 as found at Upware and Brickhill are characterised by being pre- 

 served in oxide of iron (Limonite) and never phosphatised as those 

 of the Kimmeridge clay and others are. 



The Oxfordian species include: 



Ammonites biplex (d'Orb. non Sby.); Area concinna ; Phola- 

 domya Phillipsii ; Modiola bipartita; Rhynchonella varians, var. 

 social is. 



