Suffolk, although we have good lists of special localities. That such a division exists in Suffolk, both lithologically and palaeontologically, there can he no doubt ; and that it is to some extent maintained in Norfolk is probable. But in Norfolk it is not always easy to show the line of separation, and it becomes a question whether the differences in the Molluscan fauna are not differences produced in the fauna of one period by local conditions of sea-bed, depth, and fresh waters. There is no doubt that the lower beds, both in Suffolk and Norfolk, are more-shallow-water deposits, and that, in the latter county especially, they contain a large freshwater element ; while the upper beds, with greater depth of water, show the Mollusca less drifted, more in situ, and as having been under the influence of colder currents.
Not only, however, do the upper and lower beds differ, as at Bramerton, but the same division shows marked differences in different pits. Thus the lower bed in the pit east of the one on the common has been named by Mr. Reeve the Scrobicularia-bed, from the abundance of that shell, of which only one or two fragments have been found on the same level in the adjacent pit, while Trophon clathratus, Diplodonta astartea, and Tapes aureus have been found by Mr. Reeve in the former pit and not in the pit on the common. So Cardium edule and Littorina littorea are rare at Aldeby, but are very common at Beccles (section of town well), while of the Cerithium tricinctum, also common at Beccles, only one specimen has been found at Aldeby. In the same way the fossils from the same beds at Thorpe, Bramerton, Postwick, Coltishall, and Horstead present marked differences of grouping and in relative numbers. So, as is well known on our own coasts at present, the distribution of the Mollusca presents rapid variation. Amongst other instances, Mr. Jeffreys states that the Tellina balthica abounds in Swansea Bay, but that not a single specimen is to be found at Oxwich Bay, only nine miles distant. It is possible, therefore, that the differences found to exist throughout the Norfolk and Suffolk areas are more or less dependent on these causes — that Thorpe (Suffolk), "Wangford, Thorpe (Norwich), may represent old lines of coast or shingle-hanks in the old sea, while Sizewell, Southwold, Beccles, Aldeby, and Bramerton may represent synchronous deeper- water deposits. Difference of depth is also probably the cause why the fauna of Aldeby is so much richer than that of the upper division at Bramerton, and why so many of the older Coralline-Crag species reappear. I have therefore, for the present, taken the two divisions together ; and this gives the following result : —
Total number of species recorded in the Norwich Crag 179
Deduct land and freshwater species 24
— doubtful and varieties 16
40 139
