A considerable number of these shells have been found living at depths ranging down to 100 and even 150 fathoms; our knowledge of the range of the others is, for the most part, merely negative.
From the foregoing list it appears that the Bridlington fauna consists of 70 forms of mollusca. Of these, after discarding all with regard to which there is any doubt, either as to their occurrenceat Bridlington or as to their representation in the Crag, but including the distinct variety of Trichotropis borealis, no less than 19 are unknown to the Crag.
These 19 comprise 13 purely Arctic, 1 British and Arctic, 1 British, 1 British, Arctic, and Southern, and 1 North-American, and 2 not known as living.
The mollusca hitherto obtained from the Middle Glacial, i. e. from the sands and gravels which, overlying the Cromer contorted drift in the north of Norfolk, pass under the great chalky Boulder-clay of which I have been speaking in other parts of that county and in North Suffolk, in Essex, Herts, Buckingham-, and Leicester-shires, and some other localities, comprise 63 forms, all but one of them collected within a radius of a few miles around Yarmouth, from the sands (where they are in situ) between the contorted drift and the great chalky Boulder-clay. Of these 63 forms, the foregoing list shows that, besides the 2 apparently new forms and perhaps the shell referred to Mangelia linearis, there is only one, Tellina balthica, which does not occur in the Crag, and also that, with the exception of this shell, not one of the 19 peculiar shells of Bridlington are among them.
The fauna of the Lower Glacial has been obtained from the thick body of pebbly sands which forms the base of, and is extensively interstratified with, the Cromer Till, and of which, in fact, the Cromer Till is itself only a local modification. The few worn and fragmentary examples which this till, and the contorted drift that overlies it, have yielded to a search so diligent that it has been carried on for years, all over Norfolk and North Suffolk, are all readily recognizable as belonging to the commoner species occurring in the pebbly sands. This fauna comprises, as the list shows, 35 forms; and though I fully expect that further search will augment the number of forms from the Middle Glacial sands, I fear, from the time which my father and myself, as well as others, have devotedto the search, that the Lower Glacial beds will not yield any considerable addition to the number of species which we have obtained.Of these 35 forms also, there is only one, the same Tellina balthica, which does not occur in the Crag; and, with the single exception of this shell, there is also the like absence of all the 19 forms peculiar to Bridlington that obtains in the case of the Middle Glacial.
Further, of the 63 Middle, and 35 Lower Glacial forms, 26 are common to each of these two formations, so that there remain 72 distinct forms yet obtained from the two together to set against the 70 of Bridlington. Now it is most remarkable, as the converse VOL. XXVI. PART I. H
