194. SUPPLEMENT TO THE CRAG MOLLUSOA, 
faunas were repeated.’ After this, and at probably the very last geological period that we. 
recognise in Britain, the two areas were once more divided by an isthmus between Kent and 
France, which enabled the southern fauna to range up the British Channel without inter- 
mingling with or overlapping that of the Arctic province. 'This was the period of the Selsea 
mud bed in which a considerable and well-preserved fauna of southern affinities occurs—a 
fauna which has its living analogue on the southern coast of England, and in which are 
some few species that now occur only on the Lusitanian or Mediterranean coasts. Besides 
all this, the exceptional character of the Scandinavian fauna, due, it would seem, to that 
drift of warm water, which keeps the bays and fiords of Norway free from ice, forms. 
another complicating element in the question ; while the influence of vast stretches of 
abyssmal water supporting Mollusca (of which some have been hitherto considered 
characteristic of Mediterranean areas, and others of northern and even Arctic areas) in 
intermingling distant faunas has yet to be elucidated. 
It thus appears to me that if any of these past periods could with propriety be termed. 
“the cradle of the British Mollusca,” it would be rather the Red than the Coralline Crag. 
Strictly speaking, however, it would, owing to the alternate interminglings and separa-. 
tions already referred to, be misleading to refer to any one of these past periods as 
having formed the cradle of the British Mollusca. In speaking, therefore, of the Coralline 
Crag fauna having its affinity with that of the Mediterranean, I mean merely to imply 
that the major part of the living species of the Cor. Crag are of Mediterranean habitat, 
and that of such among this part as are common to the Mediterranean and British areas, 
the greater number are more abundant in individuals in the Mediterranean than they are 
in the British waters; and that, so far as conditions of temperature can be inferred from 
a Molluscan fauna, these conditions during the Coralline Crag period would seem to be 
nearer to those of the seas of southern Europe and of the Azores than to those surrounding 
the British Islands. 
According to the analysis of my Synoptical List (post, page 219), there are 205 species 
of Gasteropoda and Bivalvia living in the Mediterranean which are identical with Coralline 
Crag species, among which there are 51 that are not known in the British seas. On 
the other hand, there are 20 species of British Mollusca in the Coralline Crag that are not 
living in the Mediterranean. It should not be forgotten that the most abundant, and 
therefore most characteristic species of the Coralline Crag, such as Oardita corbis, Cardita 
senilis, Limopsis pygmaa, Ringicula buccinea, and others, are southern species unknown 
to British seas ;. and that among the 154 Coralline Crag species occurring both in British 
and Mediterranean waters there are many which are really characteristic Mediterranean 
shells, and are only marked British in consequence of some rare occurrences, due to the 
1 The remarkable molluscan fauna, extracted with difficulty from the middle Glacial sands, and still far 
from complete, throw much light on this history. Several Coralline Crag and Mediterranean species, of 
which we get no trace in the Red Crag, or at least in the newest part of it, or Chillesford bed, again making 
their appearance, though the specimens have evidently travelled far along the bottom. At this period the 
great Glacial submergence fully set in, if, indeed, it did not attain its maximum. 
