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
