1869. ] SEARLES WOOD—BOULDER-CLAY. 97 
A considerable number of these shells have been found living at 
depths ranging down to 100 and even 150 fathoms; our know- 
ledge of the range of the others is, for the most part, merely nega- 
tive. 
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 occur- 
rence at Bridlington or as to their representation in the Crag, but 
including the distinct variety of T'richotropis 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, 7. e. 
from the sands and gravels which, overlying the Cromer contorted 
drift in the north of Norfolk, pass under the great chalky Boulder- 
elay 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 zm situ) between the contorted drift 
and the great chalky Boulder-clay. Of these 63 forms, the fore- 
going list shows that, besides the 2 apparently new forms and per- 
haps the shell referred to Mangelia linearis, there is only one, Tel- 
lina 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 Brid- 
lington 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 
earried 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 de- 
voted to the search, that the Lower Glacial beds will not yield any 
considerable addition to the number of species which we have ob- 
tained. 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 
