32*2 RELATIVE AGES OF THE CRAG 



tary examples of some other species which are either quite 

 peculiar to the Norwich beds, or are recent species never found 

 in the red crag. Nevertheless, it is worthy of remark, that if 

 we reduce the list of marine shells by ten, on the ground of 

 doubts entertained respecting the authenticity of the species 

 above enumerated, we then find nearly the same per-centage of 

 recent species in the marine list as is obtained from that com- 

 prising the land and freshwater shells, namely, 60 per cent. 

 In regard to the nineteen land and freshwater shells, there is 

 no possibility of any individual having been washed in from 

 the purely marine crag of Suffolk ; so that when we have ob- 

 tained a large number of these, they will yield the safest test 

 of the analogy of the Fauna to that now existing. 



Norwich Crag Older Pleiocene. — To whatever view we may 

 at present incline respecting some of these doubtful shells, the 

 Norwich crag will still be referable to some part of the older 

 pleiocene period, according to the classification which I have 

 adopted in the ' Principles of Geology,' while the red and co- 

 ralline crag of Suffolk will each belong to different parts of 

 the miocene epoch. 



It would be foreign to the chief object of the present pa- 

 per if I were to enter into any details respecting the fossil 

 shells of the red and coralline crag, the examination of which 

 could not have been accomplished without access to Mr. 

 Wood's collection, where almost every species is illustrated 

 by abundance of individuals. Assisted by Mr. George Sow- 

 erby and Mr. Wood, I have convinced myself that out of 345 

 species of coralline crag shells, 67 are identical with recent 

 species, being about 19 per cent. ; while out of 230 species 

 from the red crag, 69 agree with living species, being in the 

 proportion of about 30 per cent. 



It is curious that a large proportion of the recent shells 

 found in the coralline crag, are neither met with in the red 

 crag nor in the Norwich formation. They disappear in the in- 

 termediate period, which may be attributed principally to the 

 fragile nature of many of these shells, and in some cases to 

 their having been peculiar to deep and tranquil water. If 

 they should hereafter be detected in beds strictly contempo- 

 rary with the red crag, it by no means follows that they would 

 alter the proportion of 30 per cent., because with them we 

 might expect to bring to light a great number of extinct spe- 

 cies, some of which would probably agree with extinct spe- 

 cies of the coralline crag, whilst others would be peculiar to 

 the red crag. 



Newer Pleiocene Deposits in Norfolk. — It also appears, 

 from an examination in which Mr. Wood, Mr. Sowerby, and 



