102 INVERTEBRATA. 



OsTREA (Gryphea) dilatata, (Sby.) ? 



Ostrea dilatata, Sowerby, Min. Con. t. 149, fig. 1. 

 „ controversa, Roemer, Nordd. Ool. pi. 4, fig. 1. 



I cannot detect any characters of distinction between the 

 Upware and Potton Gryphites and the Oxfordian G. dilatata. Both 

 the typical and the expanded forms occur, the latter most commonly. 

 They measure respectively 5x3 inches, and 4x4 inches. 



There being so many ' derived ' fossils in the Upware deposit 

 the suggestion is spontaneous and natural that this shell is of 

 that nature having been washed out from the Oxford Clay of the 

 old shore line and buried in again with true Neocomian fossils. 

 Such was the opinion of Mr J. F. Walker; but I am convinced 

 with Professor Seeley that this is not the case, but that this 

 oyster is really a native of the deposit. Both at Upware and 

 Potton the condition of the shell is precisely that of the true 

 natives and quite different from that of any of the derived shells. 



Moreover, although the Oxfordian Ostreae are almost invariably 

 covered with Episites (Serpula, Ostrea, and Polyzoa), there are 

 no such Jurassic Episites upon the Upware Ostreae. 



But although the Oxford Clay and Upware shells are thus 

 inseparable, we cannot, in my opinion, be justified in coming to a 

 certain decision that they are the same species until they are 

 found in some of the intervening rocks of the gap which now 

 separates them in geological time. 



It is a curious point that, excluding the Ostrea frons type, 

 the Upware Ostreidae would lead the palaeontologist to believe 

 he was amongst the Jurassic rocks — Lower Kimmeridgian or 

 Upper Oxfordian or rather Ampthill Clay. For besides the 

 species now under discussion, there is in the Woodwardian 

 Museum a shell identical in shape with Ostrea deltoidea ; and the 

 little Exogyra conica and E. spiralis are undistinguishable from 

 E. nana of the Kimmeridge Clay and Coral Rag. 



Ostrea frons (Park.), var. macroptera (Sowerby). 

 Ostrea macroptera, Sowerby, Min. Con., pi. 478, figs. 3 — 5. 



Ostrea frons, Parkinson (Org. Rem. III., pi. 15, fig. 4), is the type 

 of a beautiful group of oysters which first appeared in the Upper 



