48 RELATIONS TO OTHER BRITISH DEPOSITS. 



well described by MM. Brodie 1 , Seeley 2 , and Walker' and its 

 close connexion with the Upware bed has been recognised by all. 



The indigenous fauna of Invertebrates at Potton is scanty, but, 

 so far as it goes, is the same as that at Upware. There are 21 

 species in common, mostly (13) Lamellibranchiata, including Cyprina 

 Sedgwicldi and Plicatula Carteroniana; also there are seven species 

 of JBrachiopoda, all of them Upware types : but all these fossils 

 were rare at Potton though so very abundant in the other two 

 districts. 



That the two faunas, Upware and Potton, do not agree even 

 more precisely with one another is doubtless due in great part to the 

 different physical conditions which obtained in the two areas. Pot- 

 ton was no doubt nearer to the shore line of the ancient Neocomian 

 sea than were Upware and Brickhill, and therefore the land animals 

 are much more abundant in the former place. Also a further 

 difference was due to the near presence of a great calcareous mass 

 (the Coral Rag) at Upware, which was wanting at Potton. A 

 number of important differences have resulted from this fact of the 

 abundance of carbonate of lime at Upware, for this served not only 

 to favour the growth of animals with calcareous supporting struc- 

 tures, but also furnished excellent matter for their petrification and 

 permanent preservation. At Potton, where the lime was scanty, 

 shell life was restricted, and a further result is, that even of such 

 calcareous exuviae as were buried in the bare sands, many have 

 doubtless since been destroyed by the free percolation of water. 

 Such indigenous species as do occur at Potton are now preserved 

 in oxide of iron (Limonite), a mineral which does not readily serve 

 for the mineralization of fossil remains. We have thus sufficient 

 evidence of the very close relationship of the Upware, Potton, and 

 Brickhill beds. 



Beyond Brickhill, passing southwards by Oxford, we next meet 

 with a number of isolated sand-hills which yield only the most 

 scanty traces of fossils ; and the first place that arrests us by its ex- 

 cellent sections and rich fossil remains is Farringdon, in Berkshire. 



The Farringdon sponge bed is a well-known collecting ground 

 and an old battle-field of geologists; it has been well described 



1 Geological Magazine, Vol. in. page 153. 



2 Annals and Magazine of Natural History, I860, 1867. 



3 Ibid. 



