Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/492

 The four Corals, the Bryozoa, and the Foraminifera found in the Red Crag must be looked upon with doubt in presence of its richer neighbour the Coralline Crag, which has evidently contributed so largely to its stock*.

Mollusca. — Mr. Searles Wood describes in his monograph 239 species of shells. Of these he considers that 53 species are probably derived from the Coralline Crag, leaving 176 proper to the Red Crag†. A certain number of these latter were formerly considered to be extinct species; but the important researches in deep-sea dredging carried on of late years by Mr. Gwyn Jeffreys and others have shown that many of the species formerly supposed to be extinct still exist in the deeper parts of the Atlantic and Mediterranean. Many species, also, which were considered to be distinct are now regarded as varieties.

With a view to the modifications in the relations of the Red-Crag fauna to the recent one which these changes render necessary, Mr. Jeffreys has kindly gone through the list I had tabulated from Mr. Wood's monograph, added some new species, eliminated the varieties, and identified the species found to be now living.

On the question of the species which are to be considered extraneous, there is still considerable difference of opinion. Mr. Jeffreys justly states that "the question of which species may be extraneous or derivative is at present in an unsatisfactory state." It is a difficulty which has been felt by all observers, from Mr. Charlesworth to Mr. Wood. For my own part I should be almost disposed to regard the greater number of the species found in the Crag of Suffolk and not in the Crag of Norfolk as derived from the Coralline Crag. So large a proportion of the latter has been destroyed, and, as it were, incorporated into the Red Crag, that I can conceive even certain species of the older deposit being now found only in the newer one. It would, however, be too much to assume such an origin for so large a proportion of these fossils. In the following list I have considered the fact of any species being found also in the Norwich Crag a sufficient reason for removing it from the list of extraneous fossils. As a rule I have also regarded the species found at Walton t, together with those occurring in the Sables

from the Red Crag of Waldringfield, which he has named Solenastroea Prestwichii.
 * Prof. Duncan has recently described (Geol. Soc. April 1871) a new coral

† Mr. Wood now reduces the number of extraneous shells to 25 ; and Sir Charles Lyell reestimates the extinct species as under : —

Total Number. Not known as living.

Percentage of shells not known as living.

Bivalves 128 31

Univalves 127 33 25

Brachiopods 1 1

(See ' Student's Elements,' p. 178.)

‡ The circumstance even of a fossil occurring at Walton does not exclude the possibility of extraneous derivation ; for I have found fragments of thin limestone from the Coralline Crag even there.