Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/631

 types, having then flourished in considerable numbers. In the Crag, all the Mammalia, except possibly the Arvicola, belong to extinct species. In the Forest- and Elephant-bed, three species pass in from the Crag, associated with thirteen other extinct species, and with six living species of Mammalia. On the other hand, the shells, whether freshwater or marine, so far as we can judge from the limited number yet known, are of existing British species ; and all these, with possibly one or two exceptions, are species which are continued up from the underlying Norwich Crag. The break in time, therefore, between the Crag and the Westleton Sands and Shingle is probably not inconsiderable.

These conclusions, whilst they agree in part with those of Prof. E. Forbes, Sir Charles Lyell, Dr. "Woodward, and Mr. Searles Wood, with regard to the gradual lowering of the temperature from the period of the Coralline Crag to that of the Forest-bed, differ from previous results in the proportion of recent to extinct species, showing a much closer approach to the existing fauna than before was estimated to exist. In arriving at this conclusion I have had the valuable assistance of Mr. Gwyn Jeffreys, whose researches in the seas of Europe have done so much to make us acquainted with Crag- species supposed to have been extinct, and with the variations produced by geographical distribution, nature of ground, and depth of water. As an instance of the geological bearing of these considerations, I will quote some remarks of Mr. Jeffreys * in speaking of Mactra solida : — " I regard Mactra truncata as the littoral or shallow-water and southern variety, and M. elliptica as the deeper-water and northern variety of one and the same species." " Every conceivable gradation of shape and solidity may be seen in a recent state ; and the union of M. solida and M. ellipiica is cemented by paleontological evidence." " I may also observe that when M. solida gradually finds its way into deeper water than it had been accustomed to, the shell becomes more slender and glossy although nearly of the usual size. It has then all the appearance of M. elliptica." He also alludes to the still greater difference in Buccinum undatum taken at low water and at depths of from 70 to 80 fathoms, as well as to the case of Venus gallina and other bivalves. It is upon evidence such as this that Mr. Jeffreys has arrived at the conclusion that so many of the Coralline and Red-Crag species are to be regarded merely as varieties.

In my former papers the conditions under which the Coralline and Red Crags were formed, together with the mode of distribution and relations of the fauna, were investigated. It was shown that at the very commencement of the Crag-period a degree of cold prevailed severe enough to give rise to the transport by ice into the Coralline- Crag sea, not only of flints from the neighbouring chalk shores, but

pliocene shells.
 * British Conchology, vol. ii. p. 418. This work is full of remarks interesting to the geologist on the range and habits of most of our Pliocene and Post-