456 Mr. S. V. Wood's Catuloyut* of Shells from the Crag. 



some forms in my own cabinet. Mr. Lyell and Capt. Alex- 

 ander have kindly furnished me with the list of shells from the 

 mammaliferous crag*, while those from the red and coralline 

 are what my own cabinet contains, every species of which I 

 have myself collected. 



With respect to the utility of this Catalogue as one means 

 of arriving at the relative ages of the three deposits to which 

 it has reference, it should be borne in mind that some amount 

 of error may arise from the imperfect state or altered condi- 

 tion of many fossil species ; some modifications in the several 

 proportions of extinct to recent species may also arise from 

 future researches both in recent and fossil conchology, a source 

 of error mentioned by Mr. Charlesworth. And in applying 

 the percentage test to these tertiary beds, allowance should 

 be made for the absence from the red and mammaliferous crag 

 of certain species found in the coralline and also inhabiting 

 our own seas, amounting to nearly ten per cent., many of them 

 minute and fragile. Mr. Lyell, in his paper upon the relative 

 ages of the Crag of Norfolk and Suffolk, * Mag. of Nat. Hist.,' 

 1839, p. 322, says, " If they" (the recent species just referred 

 to) " 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 thirty per cent., because with 

 them we might expect to bring to light a great number of ex- 

 tinct species which would probably agree with the extinct spe- 

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

 the red crag." Now on this point I must beg to dissent from 

 Mr. Lyell, as I consider a part, at least, of these recent spe- 

 cies should be allowed for in our estimates, since we know 

 they must have existed through the intermediate period ; and 

 though in our researches for these absent species we should 

 in all probability bring to light a number of extinct forms, 

 so at the same time we might expect to discover with them 

 a number of forms identical with those now living in our own 

 seas but not known in the coralline crag, and that we have 

 as much reason to expect in our new 7 discoveries that the ex- 

 tinct species should not much exceed the proportionate num- 

 ber of recent not found in the older bed, as that those absen- 

 tees should be the only recent species that we may hereafter 

 discover. We have not however yet arrived at that know- 



* Dr. Buckland states in his Anniversary Address to the Geological So- 

 ciety for 1840, p. 236, that Mr. Lyell proposes the term "Norwich Crag" 

 for the newest of the three crag deposits, but I have employed throughout this 

 catalogue Mr.Charlesworth's name " Mammaliferous Crag," which he gave 

 to this formation in the year 1836, and which has consequently the right of 

 priority. (See Proc. of Brit. Assoc. for 1836.) 



