PRESTWICH CEAG-BEDS OF SrFEOLK AND NORFOLK. 475 



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 LyeU, 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, show- 

 ing 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 acquaiated with Crag- 

 species supposed to have been extinct, and with the variations pro- 

 duced by geographical distribution, nature of ground, and depth of 

 water. As an instance of the geological bearing of these consi- 

 derations, I will quote some remarks of Mr. Jeffreys * in speak- 

 ing of Mactra solida : — " I regard Mactra truncata as the littoral 

 or shallow-water and southern variety, and 31. 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 31. eUipilca is cemented 

 by palseontological 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 3£. ellip- 

 tica." 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 con- 

 clusion that so many of the Coralline and E.ed-Crag species are to be 

 regarded merely as varieties. 



In my former papers the conditions under which the Coralline and 

 Ked 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 



* British Conchology, vol. ii. p. 418. This work is full of remarks interest- 

 ing to the geologist on the range and habits of most of our Pliocene and Post- 

 pliocene shells. 



