CONCLUDING REMARKS. 195 
perseverance of our naturalists in dredging the extensive stretch of sea which, extending 
over 12 degrees of latitude, surrounds the British isles. In making these comparisons, 
moreover, it should not be overlooked that we contrast results obtained from a patch of 
fossil sea bottom only a few yards square in Suffolk,’ with results obtained from the vast 
area surrounding the British islands. If instead of this a comparison could be instituted 
between the Coralline Crag fauna and that which might be obtained from an exhaustive 
dredging of the bottom of the North Sea off the Suffolk coast, my belief is that the exotic 
character of the Coralline Crag sea would become much more apparent. 
How far during the ‘intercourse which has gone on by ships between Britain and 
the Mediterranean and Lusitanian coasts for 2000 years past, Mediterranean and 
Lusitanian Mollusca have been introduced into British waters through the agency of the 
bottoms, anchors, and especially the ballast of ships, it would be rash to conjecture ; but 
the extent of this cannot have been inconsiderable, especially during the last two centuries. 
Whatever the degree to which this has extended may be, it has by so much reduced the 
exclusively Mediterranean and Lusitanian proportion of the Coralline Crag Mollusca 
below its real amount. Of course the same process must have had similar results in 
introducing British species into Lusitanian and Mediterranean waters. 
The numerous species and profusion of individuals of the genus <Asfarte, the 
presence of the genus Cyprina, and of such shells as Zrichotropis borealis and 
Glycimeris angusta, represent, on the one hand, what would be urged in support of the 
arctic and boreal features of the Coralline Crag fauna. On the other hand, the profusion 
of such species as Limopsis pygmea, Cardita senilis, Cardita corbis, Ringicula buccinea, 
Woodia digitaria, and Dentalium dentalis, and the occurrence of the sixteen genera 
presently enumerated, represent the Mediterranean features. Besides these we have the 
tropical forms, Pyrula and Pholadomya, which were probably present as lingerers 
from those periods anterior to the Crag which are clearly shown by their fossils to 
have been more and more tropical as we recede in Tertiary time. Although the genus 
Astarte among recent shells is looked upon as indicating boreal conditions in our present 
seas, inasmuch as only one species lives in the Mediterranean, yet as we recede in 
geological time this indication becomes weakened if not negatived altogether, and a proof 
is afforded that the genus did not in older Tertiary periods originate in icy seas. ‘Thus in 
the Eocene of England we have no less than four species of this genus in association with 
such an undoubted tropical Mollusc as the Nawtilus; and of these, two are from the 
London clay, the climate of which is proved to have been warm not merely by its Naud, 
but by the tropical character of the vegetation yielded by the Sheppey deposit. In 
Mesozoic formations the genus goes back in some abundance of species as far as the Lias 
in association with gigantic reptilia, through climates indicated by the Purbeck vegetation 
1 To any objection that such a dredging would not disclose the contents of the North Sea bed vertically» 
I reply that there are but very few Coralline Crag species which I have not found within a vertical range of 
three feet in the stackyard pit at Sutton. 
26 
