CONCLUDING REMARKS. 193 
author of the ‘ Brit. Conch.’ differs from us,' and some time ago expressed his opinion that 
the Cor. Crag sea was the cradle of the British Mollusca. Now, although a number of 
Cor. Crag species that were not known to live so far north as Britain have lately been 
discovered living in our seas, this does not appear to me materially to affect the inference 
of the authors of the ‘ Brit. Mollusca’ and of myself, because Molluscan, hke other faunas, 
overlap each other, and species may yet linger as rareties in areas where they have long 
ceased to predominate. Similarly several Cor. Crag species have been now found to be 
denizens of Arctic seas; some few indeed are said by Mr. Jeffreys to be, so far as yet 
known, exclusively so,’ while others range from Arctic seas down into British waters. We 
must remember that since the Corailine Crag period we have had great geological changes, 
the arctic conditions of the glacial period having fallen upon Britain, and again given way 
to our present temperate climate. Moreover, at the period of the Coralline Crag the 
Mediterranean and South European area was probably connected more directly with the 
Arctic than it now is, but the geographical changes which intervened between this and 
the Red Crag period appear to have produced an interruption of such direct connection, 
by giving rise on the east of Brita to a land barrier which shut off the Red Crag sea 
from the south and left it open to the north; the occurrence of a characteristic Red Crag 
Molluscan fauna fossil in Iceland, with some traces of it midway about Aberdeen, 
showing the extension northwards of this sea. By these means, as it seems to me, a 
part of the Cor. Crag species was induced to retire southwards and died out in Britain, 
while the other part survived there and some new importations came in. It is as probable 
that some of our present northern forms originated in and have migrated from seas far to the 
southward of Britain, as that they did so from the seas to the north, since we find Panopea 
Norvegica, Mya truncata, Cyprina Islandica, Lucina borealis, Trophon contrarius, and 
perhaps one or two more that are now unknown to the Mediterranean and considered 
as Arctic-British types, fossil in Sicily in association with forms that are mainly of Mediter- 
ranean types. After this change of coast lines, to which the Red Crag was due, had wrought 
its effect, the arctic conditions of the glacial period supervened, accompanied by a consider- 
able submergence. ‘This submergence destroyed the barrier which shut off the sea of the 
Red Crag from the southward, and once more brought the sea of the north east of Britain, 
and through it the Scandinavian and Arctic areas into direct communication with the 
Lusitanian and Mediterranean seas to which the one part of the Cor. Crag Mollusca had 
retired, and the conditions favorable to an intermingling, or at least an overlapping of the 
1 «Brit. Conck.,’ pl. Ixxxix, Introduction, where it is said, “my investigation of the Crag shells has 
not led me to form the same conclusion as Messrs. Forbes and Hanley, viz. that most of these ancestors of 
our living shell-fish are of those forms which we regard as southern types.” 
2 There are three species in Mr. Jeffreys’ list of Cor. Crag shells to which the letter A alone is 
attached, signifying that these are exclusively Arctic species, viz. Velutina virgata as V. undata, Smith ; 
Cardium strigilliferum as C. elegantulum, Beck ; and Glycimeris angusta as G. siliqua, Chemn. None 
of these identifications are to my mind satisfactory and I have not adopted them. 
