48b A. J. jukes-browne's supplementary 



Nautilus I have described, but have not named, since the characters 

 of its original shelly covering are hardly indicated on the casts, 

 although these form a very common fossil in the Cambridge Green- 

 sand. 



In conclusion, I may state that I regard the description of any 

 of these species as of far less importance than the recognition 

 among the fauna of other previously known species, and the demon- 

 stration of the identity of the separately named forms. 



A few errors and misapprehensions which had crept into my 

 former notes are now corrected. 



Regarding the general conclusions arrived at in my former paper, 

 I will only say that my opinions have been confirmed by subse- 

 quent experience during the progress of the Geological Survey in 

 Cambridgeshire ; the only point in which I am disposed to alter 

 the views previously expressed is with regard to those fossils which 

 are preserved in light phosphates. Similar nodules and fossils 

 are found in the uppermost beds of the Gault both at Folkestone 

 and in Bedfordshire ; and I now consider them to have been derived 

 from this horizon, very few, if any, of the phosphate nodules having 

 originated in the bed where they are now found. I regard, there- 

 fore, a still larger proportion of the Cambridge fossils as derived, the 

 invertebrate fauna of the Greensand itself being very small and 

 only numbering 36 species. 



CEPHALOPODA. 



Belemnites plexus, Blainville. 



This name was admitted into my former list in consequence of 

 three ot four specimens being so named in the Woodwardian 

 Museum. A subsequent examination, however, in company with 

 M. Ch. Barrois, of Lille, has convinced me that they do not belong 

 to this species, since none of them exhibit the peculiar truncation 

 of the alveolar end, which is its great characteristic. They bear 

 most resemblance to large specimens of B. ultimus or attenuatus. 



Nautilus arcuatus, Desh. 



Nautilus arcuatus, Desh. Mem. Soc. Geol. France, vol. v. pi. xviii. 

 ,£g. 1. 



This species is described as inflated towards the middle, with 

 rounded and subangular back, without any umbilicus ; chambers as 

 wide as high and very sinuate (tres-arquees) ; siphon situated a 

 little below the middle ; test thin and nearly smooth. 



It somewhat resembles JV. Fleuriausianus, but has still more 

 enveloping whorls, different chambers, and a differently situated 

 siphon, that of N. Fleuriausianus lying close upon the inner whorl. 



Several casts agreeing in every respect with the figure and 

 description of N. arcuatus have been found in the " Nodule-bed," 

 though they are by no means common. They are easily distin- 

 guished from all others by the absence of any umbilicus, and by 



