[Plate XXI.]
In a paper read before the Society in January 1875, I discussed the relations of the Gault and Chalk Marl in Bedfordshire and South Cambridgeshire, and endeavoured to show that the fossil contents of the so-called Cambridge Greensand are for the most part remaniés and have been derived from the upper beds of the Gault.
The conclusions arrived at were briefly these:—that the Upper Greensand does not extend further in a north-westerly direction than West-end Hill near Cheddington in Bucks; that stratigraphically the Cambridge Greensand is of no importance, being a mere nodule-bed at the base of the Chalk Marl; that it rests unconformably upon denuded Gault; that a greater portion of the fauna belonged originally to the Upper Gault; that the remainder, really belonging to the bed itself, are species proper to the Chalk Marl rather than to the Upper Greensand.
A full list of the species then known as existing in the formation was appended; and the derived forms were therein separated from those presumed to be indigenous to the Greensand itself. This list was mainly prepared from an examination of the large collection of fossils in the Woodwardian Museum; and my investigations resulted in the determination of many species that were previously unnamed in that collection, and in the identification of others which had been wrongly named. All these species were separately noticed, with remarks upon their determination and synonymy.
These observations, however, were by no means intended as an exhaustive study of the fauna of the Cambridge Greensand. Many specimens remained which it was not then possible to identify; and some of them appeared to be altogether new and undescribed; the consideration of these was purposely deferred until I should be able to obtain more information regarding them.
Two years have now elapsed, during which I have had the opportunity of inspecting many more specimens both from this and from other Cretaceous beds; and I am now better able, therefore, to offer some remarks upon these forms.
At the same time it is with considerable hesitation that I venture to name and place on record four species which do not appear to have been hitherto described; for I consider the establishment of such new forms as a very serious matter, and one upon which too much care cannot be spent.
It was, I think, Professor Forbes who remarked that few are aware how many species have been named from single and often broken specimens. Now it is obvious that such species cannot be
