Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/375

362 region of western Europe, as to cause an almost entire disappearance of the then marine fauna.

In the lowest, or coralline crags, however, we have presented decided relations between our existing marine fauna and that which then occupied the area of the south-west of England. The number and state of preservation of the Mollusca, Radiata, Cirrhipeda and testaceous Annellida, found in that formation, enable a very complete comparison to be instituted; and the elaborate catalogues of Mr. Searles Wood, drawn up with the greatest care and judgment, furnish materials for our inquiry, such as are not often at the disposal of the palæontologist. From Mr. Wood's researches, it appears that out of above 340 species of testaceous mollusks, found in the coralline crag, 73 are now known living in the British seas; among these are 23 which are found fossil in the northern drift, or Newer Pliocene beds of Britain. With the exception of Trichotropis borealis and Lottia virginea, both northern forms, the remainder consists of such British species as chiefly range south of Britain, in almost every case extending to the Mediterranean. The coralline crag mollusks not now living in the British Seas are either extinct forms, or species now commencing their northernmost range south of Britain: with them is associated Cancellaria costellifer, only known fossil in Europe, but still liviqg on the northern shores of the United States. The zoophytes of the coralline crag present similar phenomena. Out of 57 species most carefully examined by Mr. Searles Wood, 18 or 19 are existing British forms: these are associated with polypidoms of a more southern character, including species of Balanophyllia, Cladocara, Fascicularia, Theanoa, Hornera, Lunulites, Fungia, &c., of which we have now no representatives in our seas.

In the Red Crag there are about 260 species of testacea, out of which 60 are now known alive in the British Seas, a larger proportion than in the coralline. The increase, too, of northern forms, is very marked, 41 out of the 60 being species which occur fossil in the "northern drift." Of these, 19 are common to the coasts of Britain and America, and are associated with four species now known living only in the Arctic or Boreal-American Seas. The American Cancellaria also lived in our area during this epoch. All the British species found in the red crag, which are not known in the preceding formation, are of northern origin. The rest of the red crag molluscan fauna is made up of species either southern or extinct. The number of zoophytes has gready diminished, 25 species only having been observed, 14 of which are existing in the British Seas. The number of southern forms has also greatiy diminished.

Among the comparatively few marine shells recorded as fossils of the Norwich or mammaliferous crag, we find some British or northern