52 KELATIONS TO OTHER BRITISH DEPOSITS. 



of little importance in such a series of coarse and littoral deposits 

 as these where rapid change and great variety are the general rule. 



But besides these differences our more detailed studies amongst 

 the indigenous fossils shew that there also exist other differences, 

 which I take to be of real significance, between the several faunas. 

 This is perhaps most strikingly seen in the species of Mollusca. 

 Contrasting the Up ware and Farringdon species, we find that they 

 have no Cephalopod and only one Gasteropod {Pleurotomaria gigan- 

 tea) in common, and of the Lamellibranchs only five species are 

 known from both localities. Amongst the Brachiopoda there is 

 more affinity, 11 out of the 26 Up ware species being found at 

 Farringdon. 



In the Echinodermata the distinction is also marked; and even 

 amongst the sponges it is not easy to see why the Farringdon 

 species Peronella furcata, Elasmostoma pezisa, Catagma {Manon) 

 Farringdonense and others should not have flourished equally well 

 at Upware had the two formations been contemporaneous. 



Between Brickhill and Upware we have the same kind of differ- 

 ences though not so strongly marked ; the greatest contrasts here 

 being amongst the Echinodermata and Lamellibranchiata. 



Now remembering the close correspondence in the Physical 

 conditions of these several areas, their topographical nearness to 

 one another, and the similarity of the types of life in each locality, 

 these differences are, to my mind, greater than one would expect 

 to find along such a coast line now-a-days. 



Wherever in two separate beds fossil organisms of widely 

 different types are found — say, only fishes in one bed and only 

 sponges in another, we have but poor materials for chronological 

 comparisons; but with allied forms in two deposits which were 

 formed under similar conditions, we have trustworthy data of the 

 most exact value to work upon. Now bearing these considerations 

 in mind the differences amongst the faunas of our several Iron- 

 sand and 'coprolite' beds become of such importance as to demand 

 a special interpretation. 



In working with the Brachiopoda there will also be found some 

 reasons for supposing that the Brickhill bed was of somewhat older 

 date than the Upware rock, the former having apparently been 

 the centre of dispersion for many of our special types of Neocomian 

 Brachiopoda {Terehratula, Waldheimia, and Terebratella), which 



