INTRODUCTION. XXHl 
Premising that the general condition of the specimens justifies the assumption that 
if any of the species be derivative, the whole are so, since, the most remarkable species are 
quite as well preserved as such forms as Purpura lapillus, Trophon antiquus, and Tellina 
Balthica (whose genuineness would be doubted by none), the following reasons for the 
genuineness of the entire fauna offer themselves : 
1. So far as the Crag fauna is known, it would require in order to furnish by derivation 
the Middle Glacial shells, the Coralline, Red, and Fluvio-marine Crags, as well as some other 
bed for four of them which do not occur in the Crag, viz. 7ellina Balthica, Venus fluctuosa, 
Loripes lactea, and the not unfrequent Zrophon mediglacialis, which is the characteristic shell 
of the formation. 2. Not a trace or fragment of most of the common strong shells of the 
Coralline and Red Crags has occurred. 3. The sinistral form of Zrophon antiquus,' which is 
profuse throughout the Red and Fluvio-marine Crags, and frequent in the Chillesford and 
Lower Glacial sands, is absent ; while specimens of the dextral form, and especially fragments 
of the columella and mouth showing the dextral turn, are abundant. No derivation from 
the Red Crag, in which myriads of these strong sinistral Trophons occur, could have 
taken place without their fragments being present abundantly. 4. The specimens of 
Pectunculus glycimeris which make up so large a part of the Red Crag are large shells, 
whereas this species though abundant in the Middle Glacial is mostly in the condition of 
very small individuals and fry, the largest specimen or fragment not nearly equalling the 
average size of the Crag shells. Any derivation from the Red Crag would have brought 
an abundance of these large specimens, or of their fragments, into the Middle Glacial 
sand. 5. Some of the species, such as Venus fasciata, ave, although greatly worn, and 
mostly consisting merely of the hinge portions of the shell, perhaps fifty times as 
abundant as in any known Crag bed. 
These five reasons, to which others might be added, seem to justify our regarding the 
Middle Glacial fauna as contemporaneous and not derivative ; but although contempora- 
neous, it is evidently one which did not live on the spot where it is found, since not only 
are all the specimens, with the exception of the Anomie, more or less rolled, but the 
limited extent of the fossiliferous area suggests that it was only here that a current 
bringing the shelly sand from some other part of the sea bottom impinged. Interspersed 
with these rolled and worn Molluscan remains there occur in great numbers small 
and perfect valves, always single, of the tender papyraceous dAnomia ephippium. 
These are all young shells from one eighth to one quarter of an inch in diameter ; and their 
occurrence among such a rolled accumulation suggests the idea that they adhered to 
floating bodies which were brought to the spot by the same current that swept the 
shelly sand along the bottom; their tender valves having sunk as these floating bodies 
decayed, and become intermixed with the worn bottom-travelled Mollusca. 
* See, as to the history in time of this shell, which is an indirect confirmation of the above argument, 
p. 19 of the ‘ Supplement.’ 
