REPTILIA. 77 



creatures as we should expect to have lived on the neighbouring 

 land in the Lower Cretaceous period. 



Many of the Saurians, especially the Plesiosaurs, were also, I 

 believe, native around the Lower Greensand shores ; their back- 

 bones are often excellently well preserved, and but little rolled; 

 and although the projecting structures of their neural arches and 

 the perforations for the nutritive arteries offer just such protected 

 recesses as the phosphatic substance is generally formed in, yet no 

 such phosphatic matter is found there, and the bone itself is often 

 mineralized more with oxide of iron than phosphate of lime. See 

 ante, p. 18. 



It has been argued, and is still maintained by Prof. H. G. Seeley, 

 that all the remains we find at Potton and Upware belong properly 

 to the bed in which they are found, the animals having lived at 

 the time these beds were deposited. Mr J. F. Walker, on the 

 other hand, advocated the theory that all the Reptilian remains 

 are of derivative origin, having been obtained by the destruction 

 of the Jurassic and older Cretaceous rock groups, and this has 

 been the generally received theory. But the truth is, I believe, 

 as stated above, that both these ideas are in part correct, some of 

 the species being ' derived ' while others actually lived at the 

 period. 



One of the principal means of separating these two groups is 

 found in an important difference of their lithological nature, 

 namely the ' derived ' fossils are found, for the most part, thoroughly 

 fossilized in phosphate of lime, whereas the true Neocomian 

 Vertebrate remains are preserved in oxide of iron. Moreover, as 

 might be expected, the former group has suffered much more 

 from abrasion by water-rolling than the latter 1 . Thus I have 

 been enabled to separate out two different sets of vertebrate 

 remains, the one of contemporaneous Lower Cretaceous age, and 

 the other properly of Jurassic age. 



Additional evidences of the truth of this conclusion are that 

 certain types of Plesiosaurus vertebra which occur in the Upware 

 and Potton beds are not known in the Jurassic rocks, and on the 

 other hand it is a striking fact that the large Saurian vertebrae, 



1 The collection of bones from Potton in the Woodwardian Museum cannot be 

 taken as representing their usual state of preservation, since only the best specimens 

 have been selected there. 



