INTRODUCTION. xxiii 



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 Tettina 

 BaUhica (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. Tettina Balthica, Venus fluctuosa, 

 Loripeslactea, and the not unfrequent Trophon 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 Trophon 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, are, 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 Anomia, 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 Anomia 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. 



1 See, as to the history in time of this shell, which is an indirect confirmation of the above argument, 

 p. 19 of the ' Supplement.' 



